2003 vs. 2004. . .
If you are keeping the truck a long time, mileage and condition will matter more. If you trade every couple years, the 2004 has the edge.
Finding a truck with the equipment you want matters a lot. You may not find a 2003 that is equipped right for you.
Insurance is probably similar on them. Licensing fees may be higher on the 2004 depending on the state you live in.
From what I have heard and seen, the two years are functionally identical, with one exception. The storage pod in the front center seat back seems to require the premium cloth or leather in 2003-4. The lower cushion storage pod does not seem to be available in 2004 whereas it was in the premium cloth and leather seats in 2003 (SLT and Laramie models). Be sure to check if this is important, as I might have missed something here--I have only checked out a few trucks. With storage space kind of scarce in the 2003-4 Rams, the center front seat storage areas is important to some.