In all honesty, I noticed better average fuel mileage with the 2500 trucks when pulling heavy trailers (~7000lbs) which I often do. Although my 1500 is a great truck, I sometimes wish I could carry more while pulling the trailer, so my next truck may end being a 2500 again.
Overall, I find the mileage is not affected as much as I figured it would be. The HD trucks a bit heavier, but seems to have a lower rolling resistance. I have monitored every tank of fuel I've burnt in the last 15+ years, and have the info documented with driving/weather conditions so I can run comparisons that are basically 'apples to apples'.
IMO, you can leave gas mileage out of the purchase decision between 1500 and 2500. Under similar circumstances they data is non-conclusive, as I have found bigger discrepancies between 2 trucks of the exact same model, than I've found between 1500/2500.
Good luck with your purchase, and keep us updated as to which way you went.