I've owned both. Both are solid cars, but if it's your first car, and all other things are equal (cost, condition, miles) I'd recommend the G.
Performance is no comparison - the G (even in sedan form) will clobber a Z32 in every category. Z32 values are trending up, while the G hasn't hit bottom yet.
Pick one and roll with it. You can't really screw this up. Just buy the best example you can afford. A poorly-maintained, abused car will break your bank (and your patience) in no time.