The LS-3 has a better powerband then the C-5 Z06, C-5 is a higher RPM power range motor. Also, the C-5 Z06 only comes as an FRC-fixed roof coupe, and 6 speed manual. While you can get a 430 HP LS-3 with an auto in a convertible. I prefer the look of the C-5, because I have one, 99 Coupe, LS-1. A C-5 Z06 was probably driven pretty hard, unless you get it from an enthusiast, while you can find a pampered C-6 easier.
Mods on Vettes don't increase value, so if you want one with bolt ons, you won't really have to pay much more to get all the good stuff, but it may be hurt. I like auto cars because you don't have to buy clutches, and the trans is pretty bullet proof.
My car gets 28 MPG highway at 70, 25 at 90.