Good question!
Sound wise, I have not heard it in person on a 370Z, but I have on our old 350Z..
My preference was almost split, but I had to go with the HKS system.
Looks/Style.. the HKS looked "cooler" I guess is the word. The mufflers are angled under the car, but the tips are straight. The Ti-Tips are almost identical..
Now, the main difference, I'd say between the two are the design. The Turbo Ti-C is made using 70mm piping, opposed to HKS' 60mm piping.
This is where the sound difference is noted (aside from a different muffler/design).
The Turbo Ti-C is not necessarily for those who go turbo, nor is the HKS
not a good setup for those looking to go FI either.
What it really boils down to in the end is the price you're willing to pay for the sound and performance you'll receive.
My preference.. the HKS, but only by a small margin. I remember when GReddy first introduced their Ti-C lineup.. I fell in love. At the time, we sold out of ALL Ti-C's we had in stock from Celicas and Mazdaspeed 3s to the 350Z exhausts.
Hope I answered your question.. let me know when ready to order, or have any other questions
AJ
ECSpeedSound