My advice is to ONLY get ones that are resonated.

Non-resonated pipes/cats will be far raspier.

For me its between choosing a mid pipe or the HFC's.... unfortunately every shop around here wants 5ish hours to install HFC's on my 370Z....Consider the price of the HFC's.. and the install this gets very pricey.

I have the stillen HFC on mine with the AAM shortys. I have had them on for prolly 5000 miles or so lot of highway driving and my bumper is fine, and there is no smell compared to test pipes. Sound is ******* nice throaty as **** at 3500-4500. It is quieter in the top end because i still have the restrictive stock y pipe. Im thinking of getting an exaughst shop to custom make me an x-pipe to take the place of the stock y-pipe. Anyone have experience with this?

I installed HFC two days ago. There is only minor rasp on full throttle acceleration. No rasp at mid range rpm or hiss during deacceleration.
Most important - car is alot faster! I will do dyno in few days but my racelogic shows much better acceleration times from 0 to 100 and from 100 to 200 ( kph ).
Im very satisfied.