I say go with the 2500. I don't have GMT400 1500 experience, but I can't see any downside to the 2500, other than reduced fuel economy if that matters to you. It doesn't to me. I bought mine specifically for the heavy-duty underpinnings. Daily drove it for 5 years, including in the city. Never an issue with how it rode or drove. Only reason I don't DD it anymore is because I aim to keep it forever and they use salt in the winter here. Will eventually have another newer 2500 daily if I can find one that's in good shape and not a crazy price.
Fair warning, heavy-duty components including 4L80s are mechanical devices, and they can fail. Mine was never right from the day I bought it. Couldn't put a finger on it for a long time. Eventually had it rebuilt by a reputable shop, including a shift kit, and it's a beautiful thing now.