either way you go, you'll get a nice upgrade... but it all depends on how much stiffer you want to go, stillen is designed for more of a street setup while hotchkis can be street as its lowest setting but can be used for the track. Check out this video from hotchkis to get an idea of their setup:
Nissan 370Z Sway Bars: Stock Vs. Hotchkis - Car Videos on StreetFire