The LS4 is a pretty solid engine, as is the 3800SC. Overall, the 3800 has more parts for it right now for mods but the LS4 is catching up. Trans is the major issue like you said, a little more prevalent in the GXP's due to the higher torque rating. GXP gets my vote as it gets all the other parts the GT does not - sharper looking trim, performance suspension, and that V8 growl!