Well OP im in the same position your in. It would seem the GTM kit is better for performance no doubt. If you want performance Get GTM. Worst feeling in the world is spending X amount of money and you still cant beat a full bolt on (Mind you when i say fun bolt on im speaking of cams too) 5.0, SRT8, or SS. But if you don't care about power then don't worry about it.
Right now i like the Stillen kit because you can run LTH headers with the stillen kit like user Swiss370z, And Rcz34 did. People have lost power with the GTM kit when they run LTH. Going through the fast intentions LTH thread i found this statement...
"I have never in my life ever, ever, ever, installed a supercharger on a car along with headers and lost power. The only way it is possible is if the supercharger kit is inefficient" That was a red flag for me... Im not going to remove the headers for a kit (money lost) But that's me.
I like stillen as a company and the fact that they got their kit carb legal saids a lot.
The sound of the stillen supercharger kit is louder! the GTM kit seems quiet. Both kits dont sound like what the 350z vortech kit was
GTM is more in price. And i've watch a video on youtube of a Black girl bash GTM's customer service. She ordered a system and it took for ever for her to get. She recorded conversations when she called GTM and everything else. I saw that and was turned off from the company.
Now that said im happy GTM supports our car and they know alot about the VQ motor. I hope GTM can become what AMS is to EVO. Really looking forward to the twin Charged System! But the GTM kit does not seem like its for me. but im trying to force myself to like the kit as i dont wanna be the idiot being beat by a full bolt on American muscle car lol.
Im waiting on a kit ill really love, so op really do your home work because we are all different. I.E i like the flashy look of the Stillen kit but you dont example of something that doesn't work for you which GTM kit seems to be less maintenance and a clean look that you like. So its not about what everyone else likes its about what works for you, good luck