I've had both Tetra, and the REV2 right now... There is not really much to be comparing the REV2 with the Tetra for in my opinion... the only thing that the Tetra's have that the REV2 do not is the COMBO/MULTI modes, and the analog feedback... other than that, they sound the same to my ears... the REV2 though is much more hands on, has digital FX onboard and it's synth engine has been extended with more modmatrix slots etc... and with a 16 voice REV2, the problem with voice stealing is a lot less than an 8voice paired Tetra.
Comparing it to an OB-6 I can only do when it comes to the videos I've seen of it, and comparing the synths engines. Engine wise you'd be stupid not to choose the REV2.. .it's soooo much deeper and featured than the OB-6 in any way, and it's not even necessary to compare it... this is a simple fact... the only real goal in a comparison is the filter and oscillators, and in that case, the OB.6 will win in most cases... that is where the choice becomes hard, since that should be a rather important factor.
Also how would you use the synth?, will you go deep in sound editing?, what types of sounds do you want from it... THAT is what you should make your choice around... 6voices is not much unless you generally use sounds with a relatively short release, and you go for mainly bread and butter synth sounds... if you need complex pads or other sounds, you'll quickly run into problems in the sound design... that is where the REV2's deep engine comes in handy, especially the stacking feature of the two layers is more important than you would think, and what makes truly outstanding sounds possible with the REV2... plus the addition of the internal FX which will give you a much broader stereo perspective in contrast to the Tetras... the OB-6 have about the same FX though and has two of them on it's only layer... REV2 it's ONE FX per layer which can be a little limiting compared with the OB-6 though.
So consider what you'll use it for... you already know how they sound in comparison and prefer the OB-6... if you do not necessarily need the strenghts i mentioned about the REV2 compared to the OB-6, then certainly you should think about getting the OB-6... if not... then REV2.
My 2 cent.