Up to 2002, the 1500 and 2500 use pretty much the same drivetrain, although the 2500 is available with the V10 or Cummins diesel, which the 1500 is not. Assuming you were to buy the 5.2L (318 c.i.) or the 5.9L (360 c.i.), your fuel mileage and power will be pretty much the same, assuming the gear ratios are spec'd the same.
If you aren't going to use the truck for towing a huge load on a continual basis, I'd go with the 1500. The ride quality is much better than the 2500 or 3500.
-slacker