Matt,
Do a search on "G2Z" in thread titles and go back up to 2 years, and you will hear some pretty serious praise of the G2Z. Personally, I would go with the G2Z unless you want to be able to clip it to your belt/pocket, in which case go with the C2. You definitely want the grip ring. I found that the 6P/D2 body, without the grip ring, was simply too un-ergonomical for me. My grip kept slipping forward. The grip ring is awesome! And the grip of the G2Z will be so much the better than the C2 because the C2 has a square-ish cross section due to the flat spot for the clip, whereas the G2Z is round.
If you want a 3 cell SF, definitely, hands down, go for the M3 or M3T. (Do a search, there is at least one thread on this question) I got rid of my SureFire stuff some months ago, but if I ever buy another SureFire, it would be an M3. That torch feels so good in the hand, and it looks great. It took me a little while to get used to the looks of it, but once I did, I fell in love with it. Plus, if you have the turbo head, you can use the N2 LA, which according to resident SF experts is "DA BOMB". A real nice beam. Plus the M3 has a Shock Isolated bezel while the C3 does not. If you drop that baby on it's head, bye-bye P61. If you do decide to get a C3, splurge for an M2 SI bezel. And of course, HA is worth every extra penny and then some. So don't get a non-HA C2/C3.
Just some opinions for you. Others will have a different take.