The GT500 to 370Z comparison is a very difficult one to make seeing how many differences there are just in the general makeup of the cars. 4 seats vs 2, overall size/weight, IRS vs solid rear axle, etc...
GT500 competes with the Camaro SS, SRT8, Charger, Challenger, and few others I am sure I am forgetting.
370Z was meant to compete with Corvette, Cayman, and Z4 to offer similar performance at a much cheaper price.
When it comes down to it between the GT500 and 370Z you are really just comparing lifestyles not car performance.
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09 370Z - FI TDX CBE/Stillen G3/FI HFC's are next
06 Xterra OR - 3" SL - 33" Pro-Comp MT's - Shrockworks Sliders & Rear Bumper/Tire Carrier - Stillen Cross Drilled Rotors/Pads