If you are looking at C5, you need to look 01 or newer. The older ones have an issue with the abs controller, that if it goes out you are sol. You're abs, traction control, and stability control quit working and this part CANNOT be fixed or repaced. (research this if you want too). Some say that none of these are needed, but sometimes they are nice (panic stops).
otherwise they are completely different cars, both look great and go fast.
2006 GT, Redfire
Steeda Sports, Koni's, LCA, Relo Brackets, Off-Road H, Tuned
2005 GT, Mineral Grey