I have both (5.3 V8 and 6.6 Duramax) in two trucks. Around town, hands down the 5.3 is the one to have.
The in-town mileage is very similar; on the highway, or tuggin', is when the diesel shines.
GM diesels use both DPF and DEF. DPF is the exhaust "screen" that collects soot and burns it off. Requires elevated idle and high temps. DEF is the urea-based fluid that shoots into the exhaust stream to trap soot. Must be filled periodically.
With the DPF and regen cycles, I wouldn't want to drive a diesel in town for a commuter car.