If your choice is a REV2, I would start with the 8 voice... The upgrade is not much more expensive if you want it later, but you would need to do the installation manually yourself, do not forget that (though it is quite simple, but could be daunting for a beginner maybe)... Here in Europe, it was actually cheaper to go for an 8 voice and an upgrade, then buying a 16 voice earlier on.
If you will miss the 16 voices depend a lot upon what type of sounds you would like to play... If they have long release times like pads etc. Then you will quickly want that upgrade, especially if you want to use both layers, as that will reduce the voices in half.
The OB-6 or P6 is a good synth for starters, no doubt, but it will fail in voice count and flexibility to the REV2, especially as your experience grows and you require more complexity and voicecount... The only real advantage these have are their sound, and the one knob per function interface. So be careful with your choice between these... Know exactly what you might need. Many advice for these because they like the sound more, but they probably also have more than one synth to complement it's lackings in sound engine depth and voicecount... If you are a beginner and want the most bang for the buck, the REV2 is the best in my opinion... If you are ready to learn and dig deep that is 😉