Pretty much all cat back exhausts regardless of look or sound will give you the same increase in hp which is to say about 5. That means that it's mostly a subjective choice based on what kind of look you're going for, how loud you want it to be and what material you want it to be made out of.

Personally I want something that is full stainless steel (both the pipes and the muffler), not too aggressive looking, and a bit louder than stock but not annoyingly so. Just enough to let you know it's not stock. The best exhaust that satisfies these requirements for me is the Tanabe Super Hyper Medallion. If I wanted something a bit less conspicuos I would get the RS*R Exmag but it's just a bit too quiet for my tastes. Also the DC Sports cat back is nice but I like the Tanabe SHM more.

Another really popular (and reasonably quiet) exhaust is the A'PEXi World Sport but I'm not so much a fan of it because it has pipes that are not made out of stainless steel.

So if I was in your position I'd probably get the RS*R, unless you want a little more twang to the sound and then I'd get the Tanabe SHM.