I'm assuming that you are talking about a G35 coupe? The back seat in a G coupe is more like a padded shelf than a back seat. You can't exactly fit anyone besides a kid back there comfortably. The RSX is pretty much the same way.

So basically you are comparing 4 two seaters as far as comfort. Just because they have back seats doesn't mean they are actually functional LOL.

My choice would be the G35 Coupe. The S2k is fun, but too small interior wise for me. The RSX is not very exciting to drive (even the Type S isn't all that IMO), and I don't like the outside of the 350z.