That depends on what you want. We don't have a lot of options. If you want a pre-fab cat-back exhaust, go with Ultimate Racing (they're the only one who make it). If you just want an axleback setup, go with either RRM or Ultimate. That's it. There is no difference between the RRM and Ultimate axlebacks, unless they have different tips. They both use the same model magnaflow muffler.

Search around and you'll find plenty of arguing about whether to go cat-back or not.