Dude, give the P6 a MISS. I really was disappointed back in 2016 when I had one. It barely sounded like a prophet and worse, once you're over the novelty you realise how generic and static it feels (even next to old DCO stuff I had at the time).
I've had the P6, now have the Rev 4 prophet 10 which is sublime and tonally everything the P6 wasn't. Not that I'd suggest that in this case, no you prob don't need the P10.
The OB-6... love it, but it lacks in certain areas that often makes me feel it's a kind of expensive 'nice to have', ultimately it's not a complete keeper like the Prophet 10 and Trigon 6 are.
Trigon 6? Hands down the best of the 3, runs side by side with P10 but with different tone/features, super powerful tone, real heft and bags of character that is absent on the P6. It's also much more flexible (having the 4/2 pole option is awesome as it's very useful for different sounds) and the 3rd osc, fully controllable instead of just a dumb sub osc, is a dream.... also means you get a 2nd true mod source while still having a 2 VCO synth.
Honestly it destroys OB-6 and P6 from both a power and tone perspective, but OB-6 has some very sweet things it can only do that few synths do. Trigon is a bargain vs the other 2 at similar/same prices (esp since price cut) and will give you far more reward over time as you dig out the sweet spots in the VASTLY bigger range of sounds it can do. Also the Trigon ALSO has very punchy envs and does the 'clicking' if you so wish.. it's much punchier than P6 which sounds flat while technically working fast, its very important to realise the inflexibility of the VCO mix section on the P6 - driven full it sounds terrible/hard/compressed/dull (filter sounds dull as well when you turn it down not a prob on the Trigon's 4 pole at all), so you MUST run P6 VCOs at half volume to get anywhere close to it sounding decent, but you can't then PUSH into the filter - AT ALL - while trigon can be wonderfully gentle and soft or a complete monster, pushing and twisting into the filters - BY DESIGN with additional tools like drive and feedback absent on the P6, but even without those the way it runs into the VCA/mixer sounds way better, half volume is smooth and pretty and full volume sounds powerful and rich, no 'constrained dull tone' in the trigon, it's a beast!
Trigon 6 gets my vote, I wouldn't use a Prophet 6 even if given to me for free now... that's the truth. It's just 'there' but nothing special.