If you crave the superb SEM Oberheim filter sounds, then the OB6 would be a no brainer.
Most other products from Sequential feature Curtis type filters.
I own an OB6 and Prophet REV2 and they are miles apart. Like djs123 said, the OB6 gives me goose bumps sometimes, whereas the REV2 never did. Maybe I'm an Oberheim kinda guy too?
There's a mountain of difference between the filters in a Rev2 and a P6 though, and he's asking about the P5, which I don't have but it complicates the question a bit because the OB-6 has modern features like FX and an additional voice over the P5, where the P6 is a more fair comparison.
It doesn't answer the OP's question directly (P5 vs OB6), but if I could only have one I would take my P6 over the OB6, because the versatility is greater and most of the things the OB6 does, the P6 does a little better. That said the OB6 has that Oberheim fizz to it and the SEM does have a unique sound, so I'm glad to have both.
With a P5 you get two filter choices, the vintage knob, one less voice, but 61 keys, etc.