Honestly, the only rea gain that you will see is in the difference of the diameter of the pipings of the exhausts. If they are have the same piping, most likely 2.25" or 60mm, I don't see, how you guys can say that brand X will make more power across the power band compared to brand Y.
If you are gonna be going FI in the future or really hard core N/A, then you should look into a larger diameter piping exhaust. Maybe like a 2.5" or 70mm exhaust or even a 3" exhaust for Turbo setups! But if you just have bolt ons and all the regular stuff, a 2.25" or 60mm piping exhaust will do you fine.
One may make more power than the other, but te gains are very minimal like 1hp but then it may lose power in other areas of the power band while its making more somewhere else.
The only thing that you should really be concerned about when buying an exhaust is the selection of the sound. One will sound different from another and that should be your goal in buying an exhaust. At least in my case it is.
And for the comment about going FI and having a quiet exhaust. Well, that's only true for Turbo's because the Turbo works on the exhaust side and actually silents the exhaust. Whereas for a Supercharger, which is on the Intake side, silents out your Intake, if you have one. I have had my cousin's AEM CAI on my car and that thing is silent compared to his car which is stock and its loud as hell on his. So an Exhuast for the SC will be loud too if you don't get a very nice quiet one.