I have had the both the stillen kit and the gtm kit
I had nothing but problems with the stillen kit and their support
The gtm kit made awesome power right out of the box and Sam is awesome to work with
Here is a blog post about my experience with both kits
There is dynos for both kits on the same dyno in that blog and side by side comparisons of power
Z-Car Blog Post Topic Determined to Boost: Tom’s 370z