Many will tell you GTP because of how inexpensive it is to mod, and that they can be made as quick as GXP's with some fairly basic mods. Only reason to go to GXP is if you like v8's, and don't like the sound of v6's. You will get the supercharger whine with the GTP though which can be equally as cool.

They're not too different though. I like the GXP's front bumper better, just going by looks.