You say the vette is cheaper to mod but you can do lil stuff to the GT-R and gain power as-well.
My vote is the GT-R IF you keep it between these two. Im liking the CTS-V ALOT!!
GT-R!!! Sexy not as- many people have therm as compared to vettes, and it's a whole different ball park of a car. Do something diff and get a GT-R