They're not alike at all, other than being Creed frags. It's like comparing San Fransisco to LA - same state, completely different places. You can't use one for comparison purposes to the other. GIT is a deep green/violet dark experience, Aventus is a pineapple meets Scotch Tape thing that doesn't evolve much as the fragrance dries down.
I don't prefer one over the other, although I respect the "perfumanship" of GIT much more than Aventus. GIT was a groundbreaking fragrance when it was first released. GIT is far more complex and sophisticated and doesn't play hide and seek after you've had it on for a bit - Aventus is stealthy at times, you think it has disappeared and then it starts back up. Unlike GIT, Aventus is very unisex. I've been wearing Aventus more frequently because it's the new kid on the block.