The Stillen exhaust has proven hp gains...is heavier than most, quieter than OEM, perfect fitment, willl last the life of the car (SS construction), and is pricey. If you want a more aggresive exhaust note from the Stillen...test pipes, either resonated or not, will provide it along with additional hp and a pleasant gasoline aroma at idle.
I know you didn't ask about this one...but it's the only one I know much about.
I think there is an issue with the Invidia...if I run across any info I'll add it.
edit: The first shipments of the Invidia exhaust had a 2" pipe diameter...good for N/A....not so good for F/I. Idle has been reported "lopy" without the addition of an "H" pipe. Those pre-blued rainbow titainium tips are kind of a trendy item...but that's just my opinion. Fitment is perfect and install is simple. Quiet until broken in...no rasp....aggressive at WOT.
My suggestion would be, if you're considering going F/I in the future, and the pipe diameter is unchanged, pick something else.
hth