Not entirely sure if there's dyno plots that show that divorced is better than bellmouth or vice-versa, but the Stromung DP is cheap ($195), I have the race pipe that mates to it already, and I can sell my Cobb.

Some say that having the wastegate gas stream merge in further down the DP helps with spool by having less turbulence right outside the turbine exit. This sorta makes sense, since the turbo spins due to the pressure differential across the turbine. I guess I'll find out what the butt dyno reads.

Either type is better than the "flat plate" style that the stock DPs use. (see also: Downpipe FAQ on NASIOC)