I know, late to the party, but as other people said, unless you're going big with your mods, go for a 2.5" DP. If you want aluminized, you have a lot of choices, but the only stainless 2.5" DP I've seen is ZZP's.

Avengr, bolt on replaces the entire downpipe, cat, and ubend, so you can just bolt it into the car. Weld on only replaces the downpipe, so you weld it to your stock cat and need to get the ubend removed separately if you want to.