If you want an OB6, I'd say go for it!
Only you can decide how much you'll miss the Prophet 12, but the fact that you're considering it would suggest that it probably isn't as precious to you as it might be to others.
Both models are aspirational synths IMO.
I LOVE my prophet 12 it's quite possibly my favourite synth ever, but that's not to say I don't love other synths (I do) and I guarantee the OB6 will make sounds the P12 will not & vice versa.
What it really comes down to is what sound you prefer or use the most and also whether or not you can give up the other things that make the P12 so special like the polyphony, the large OLED display & the two assignable sliders.
Sometimes you just need a change!
Also OB6's seem to hold their value pretty well, so if you were to swap your P12 for an OB6 and regretted it, I'm pretty sure you'd be able to sell it and get enough back to buy a second hand P12 again.