The 2500 comes with the 6 speed transmission " 66RFE" This transmission out performs the 5 speed as they are not the same technology, the diff gearing is not that much different and will be compensated by the 6 speed that comes with the 2500.
The 66RFE is a different beast of a tranny when compared to the 545rfe. It actually operates as a 6-speed all the time. It is basically the gear train from the 68RFE (heavy-duty diesel) transmission, crammed inside of a gas engine transmission case. Its ratios match the 68RFE:
1st gear = 3.231 ratio
2nd gear = 1.837 ratio
3rd gear = 1.410 ratio
4th gear = 1.000 ratio
5th gear = 0.816 ratio
6th gear = 0.625 ratio
Reverse = -4.444 ratio
Again this is just my opinion but the actual truck 2500 outways the 1500 when it comes to load and towing capability's, Again JMO Hopefully others will chime in to better help you decide.
You will also get better city millage with the 3.73 and be the same for highway mileage as the 3.92