If you have Motor Trend On Demand access you need to head over and watch Engine Masters Episode 60 that just posted. They do a series of tests with single 3" and dual 2.5" exhausts, which pertains to most of us.
The takeaway: if you can run a single 3" exhaust with a Y merge and an open (i.e. see-through) muffler, do it. If you have to use a turbo muffler, a single 3" exhaust is going to give up serious power. I really would have liked to see them do a test with an X or H merge pipe on the 2.5" exhaust - I think that really hurt it in comparison to the Y on the 3".
To be honest, I was in the "duals are always better" camp. I just put dual 2.5" exhaust with cats, an X-pipe, and open mufflers on my truck.
The takeaway: if you can run a single 3" exhaust with a Y merge and an open (i.e. see-through) muffler, do it. If you have to use a turbo muffler, a single 3" exhaust is going to give up serious power. I really would have liked to see them do a test with an X or H merge pipe on the 2.5" exhaust - I think that really hurt it in comparison to the Y on the 3".
To be honest, I was in the "duals are always better" camp. I just put dual 2.5" exhaust with cats, an X-pipe, and open mufflers on my truck.