I've got both - the P6 is an excellent synth with great core sound, dedicated high pass filter per voice, and good osc FM capabilities, but a more limited sound palette overall.
The Rev 2 also sounds great, and is super versatile. Its got both 4-Pole and 2-Pole Low Pass filter modes... the 2-Pole is key for getting brighter sounds (where the 4-Pole is darker). It's bi-timbral, which means you can stack two sounds, or create binaural/stereophonic patches, or keyboard splits with two sounds. It has more voices (16), meaning you can play bigger/atmospheric pads (even stacked ones), without running into voice stealing. It has a significantly deeper sound engine, with four LFOs per voice, four ENVs per voice, and a nice 16 slot mod matrix... so you can design much more complex, nuanced, animated sounds on Rev 2. In addition, the gated/modulation sequencer on the Rev 2 is very good, allowing you to further design motion into patches or sequence notes.
I personally would keep the Rev 2 if choosing only one... but as others have mentioned, it depends on what type of music you're playing and how you'll be using it.