I went for 8. There are times currently when I think 'would be great to have 16', when I'm using layered patches (I rarely use or would use split mode so 'meh'), but for layered patches there's some great sounds and obviously 4 voice poly can limit that, but in reality it's not that big a deal. I can easily get by with 4 voice for recording those parts, and most of the time I seem to prefer to just use a non stacked sound in general for my own sounds, more pure... more why I like analog anyway.
In that respect I'm glad I went for the 8 voice and have no regrets, the 8 voice price is great, while the 16 voice price is, for me, a bit too high a cost unless large polyphony is very high on your agenda (for me I'm happy with 6 voice poly on my old analogs so..), so unless you're layering then 8 voice for me is more than good. And the price makes it a very good synth for the money, if I was getting up towards the 16 voice price I may not be quite as happy paying that much and it would lose some of its value over the prophet 08 because it cost so much more.
And at least we have the add-on card option should we ever feel the need but I honestly don't think I will nor warrant that extra £500 for another 8 voices when the main point, the main guts/beauty of this synth is already there within the 8 voice, again, so long as layering isn't big on your agenda (and it may be because it can pull off some awesome layered sounds). If I was desperate for that, as they don't really interact much between layers (other than morphing style modulation if programmed that way) I could quite happily just record 2 separate patches twice into my DAW for layering.
If you play live and want that, then 16 voices is probably a much better idea.