From Sequential I've owned: Rev 2, Prophet 6, OB-6 and currently Prophet 10 Rev 4.
I want to make something clear, everyone here comparing the OB-6 to THEIR (non rev 4 non Prophet 5/10) Prophet is absolutely missing the point. It's really annoying actually that people are assuming something as weak sounding as the Rev 2 is 'comparable' to the mighty Prophet 5/10 rev 4 and are therefore in possession of anything like a valid opinion. Horseshit. Sorry.
Same goes for Prophet 6 owners who THINK it sounds anything like a Prophet rev 4 (it doesn't). From a GS thread:
"i had to do a double take when i saw your thread title, but only because i was planning on making a similar one. i'm not in the same situation insofar as i've never had the good fortune of owning a p5, but have wondered about this comparison more than once. i owned a p6 for a few months and downright hated it. "it's not the gear, it's user error" blah blah, whatever, i spent months dialling it in, it just didn't work for me... can't we just accept not every piece, no matter how universally adored, might not work for every single player? anyways, coming from the OB-8 i found the p6 downright anemic sounding and missed the organic, lush and gritty nature of the obie. i've played several p5's at friends studios and always loved that sound, as does everyone else and their granny, and definitely heard no similarity whatsoever in the p6. i'd pretty much written off dsi (having owned a few and never vibing with that sound), but the OB-6 is the one i'm willing to give one more shot as several folks whose ears/opinion i trust speak nicely about it when comparing to choice vintage synths. will be curious to see what others chime in with."
Which I agree with 100%. People HAVE to start accepting that the Prophet 6 is NOT comparable to Prophet 5 (old or new) in tone and feel. It's like a weak facsimile and sounds completely different in general use (and btw save me the comparison vids where two sounds are made to sound the same... I can pretty much get any decent analog poly to sound like any other if you're trying hard enough but the nuances of actual tone/response and 'sound quality' do NOT come through in stupid A/B videos... only time in your studio/in your music tells the story). And btw the Korg Prologue 16, at least in the VCOs (which also happen to be discreet but in this case GOOD discreet) sound far more 'VCO like' than the Prophet 6 and.. even the Ob-6. The Prologue has swimmy VCOs with texture, just like the Rev 4 Prophets (in fact I'm cloning sounds from both for back up and can get kinda close at times with both even if they are wildly different in filter type 12db vs 24 and overall operation... basically my point there is the 16 voice!!! VCO Prologue already sounds way more charming and vibrant than the Prophet 6 so I'd never buy a P6 over a Prologue! OB-6? Yeah.. different, both have their place but part of why I sold my OB-6 was cos the Prologue could copy a LOT of the 12db lowpass sounds anyway... better VCOs, filter not quite as obie as the OB-6, but actually more flexible through it's range (more prophet like).
With that out of the way, the OP was asking new rev 4s vs OB-6... well those 2 synths happen to be the best sounding analogs sequential made, by far. The OB-6 is MUCH nicer than the Prophet 6, BUT it is NOT as rich/full/deep/vintage sound as the new Rev 4s. They both steer towards different sounds, but can cross over in some areas. Usually Rev 4 or old P5's have a more raw, authentically analog tone that works great as a base materal to be further process with FX etc in your DAW. OB-6 is more 'oven ready' with built in fx, but IF you bypass said fx the raw tone isn't THAT impressive in many areas (only in a few which is why I sold mine).
What OB-6 does have going for it is a beautiful filter, generally good tone and it sounds present... as analog synths should (and as Prophet 6 never did and never could!). I sold Prophet 6 after fighting with it sound how a prophet should sound. I never missed it. So many people are deluded thinking just cos it looks like a mini P5 that it actually sounds/reacts like one. Even the polymod is off on the P6 vs the 'real' sound of Polymod on the rev 4s. Also this 'discreet' vs 'chip' BS has to end, it makes NO difference, inherently, to a synth's sound quality if something is discreet only if it's GREAT discreet (for ex my old Polysix had discreet VCOs but they sounded cheap... ) great discreet VCOs sound great, but P6.. even OB-6 doesn't have 'great' sounding VCOs.. just kinda vanilla. THe CEM (not to be confused with the weak DCOS and Weak filters in the 08/rev 2) VCOs on the rev 4's is the same legendary chip used in... Memorymoog, Obies, Jupiter 6(I think), SH-101(poss)... Futher, the filters in rev 4 (SSM and CEM basically) are spot on, both are raw yet refined, alive yet smooth, characterful yet pliable....
Prophet 6's filter is dull, just reallly dulll.... open up and it sounds cold and harsh, cut it back and it sounds lost and small. OB-6 was a much better sounding synth than Prophet 6 BUT it couldn't do as many different types of sounds (other than varities of overtly synth sounds with its bandpass filter), so neither was ideal but at least Ob-6 bought you some great tone and often beautiful sounds.
Now I have many recordings of these synths, I went back and replaced most of my Rev 2 sounds (cos they were horrible harsh and weak with hindsight) first with Prophet 6, then again with Prophet 10 where I could - the songs sound much better now. OB-6 sounds mostly can't be replaced as it's too different in my uses but you can substitue rev 4 sounds in some cases.
What is also important to realise is ONLY the new rev 4s nail the tone of a VCO analog synth, not even Ob-6 did quite (more like a really cool DCO sounding thing). So the new Rev 4s have the same vibe in the filter as the OB-6 (but different character) but MUCH BETTER VCOS (and more granular/usable polymod). SO if you imagine the range of a P6 with the mojo of an OB-6... and then DOUBLE IT - there's the Rev 4s! And the Prophet 10 in particular is awesome cos of Poly unison mode that still gives you a 5 voice stacked 4 VCO super prophet! IMO in that mode it's the best sounding prophet/synth that Dave has EVER made.. inc original rev 2s which just can't compete next to a Prophet 10 rev 4 in unison (ok maybe the original old P10 with stacked voices could compete BUT they miss out on aftertouch, filter velocity etc).
As for feel of the instruments, different leages entirely. The P6 and OB-6 always felt a bit stupid, toy like, cos of 4 octaves and I hated being cramped by the keybed (as a decent player who uses both hands and 4+ voice chords), also 6 voices was bare minimum for me but would still sometimes annoy me. For the price they felt a bit under cooked, and should def have been 5 octaves. The Rev 4s... wow.. it's worth every extra penny over the p6/ob-6 JUST for the looks/build/feel/size alone... 5 octaves, glorious keybed, beautiful BOLTED ON pot knobs, tons of space for fingers... gorgeous wood, classic stunning looks that never age and it looks every bit as good as it sounds - and it 'looks' worth the money.
Factor all of that in and I hands down recommend the Prophet 10 rev 4 over everything... I may not be so keen on the P5 cos 6 voices minimum for me, but tone and looks wise it's the same great synth so if you're ok with 5 voices... go for it!
OB-6 is still a sweet synth, with pockets of beauty, but if you're only gonna grab one synth it may not be that great as a workhorse, while the P10/5 can be (and old P5's have often been the only synth in many famous guy's studios cos they are super flexible for a proper 'vintage' VCO synth.... all those FX, stereo outs, arps, seqs etc are really stupid to put above great tone and feel... they are ok as additions but never put features over sound on ANALOG synths).
OB-6 and Rev 4 Prophets are the only synths I rate from sequential and you'd prob be happy with either, though the P10/5 would last you years and go further ultimately, the OB-6 gets a bit samey and even though it can do more than you think, it's always flavours of the same OB-6 tone, fizzy and sometimes a touch flat... but still better than the P6 which sounds lost, distant and more like a decent VA.
If you really want analog in the best meaning of the word, get the Prophet 10/5 rev 4... nothing else on the market even compares (not even moog one) let alone the weak rev 2 or the dull Prophet 6.
Ranking:
1. Prophet 10 Rev 4 (or 5 if you can hack the low poly)
2. Korg Prologue 16 (for many things) / OB-6 (for its subset it does really well/beautifully)
3.
4.
5. Prophet 6 (nah.... just nothing special - a perfunctory unit that does what a synth does... nothing special)
6.
7. Prophet 08/Rev 2 (not only weak and uninspiring but actively works against you in harsh/brittle tone - you'll waste hours of your life trying to dress it up with LFOs and mod slots to breathe life in it but still realise the tone is off and useless for most 'analog' duties and only good for cold sci fi sequences - certainly next to any other decent analog you have that will REALLY highlight how bad the REV 2 sounds as an 'analog' (hint my JX-3P destroyed it when I had one)