The downside to a 3" exhaust is you run the risk of drone. The Cobb 2.5" does not drone based on reviews here. I have a 2.5" MAP catback with a Cobb downpipe and they sound incredible together.
The Cobb catback and Map catback are very similar - both 2.5" and both very well liked on this forum.
I've heard mixed reviews of the Injen exhaust.
Could you do an Injen catback with a 3" downpipe? I thought it's 2.5" at the flange, then goes up to 3".