Power wise, under 600hp there really isnt any difference between a 3.5" and a 5", maybe a little cooler EGT's.
Sound is the only real difference between them. The whole backpressure myth is still out there and a lot of guys think you need backpressure for an engine to run right. 2-strokes are about the only engine that "needs" backpressure, pretty much all 4-strokes can be tuned to run right without it. In a duramax the turbo is all the backpressure you need, anything behind it is there to get the smoke away from the cab.
2008 2500HD LTZ, EFI Live- Idaho Rob's Tunes, 4" Magnaflow Turbo Back, Raptor 150 Lift Pump, Transgo Jr, All Season Diesel EGR delete with high flow Y-bridge, Edge Insight for gauges,