I have been able to use the maps from ut2003, so likely mods would work too.
I would have to disagree, there was a lot of extra game play added to ut2004, i would think it is a good upgrade. I bought ut2003 and was very happy with it and got a good year of game play out of it, it was my most played game, atleast on pc, used to play games more on console, but i hardly have the time or patience to play much games over and over till i completely master it. However I have enjoyed the ut series quite a bit, all have seemed worth it to me to purchase them, and so i have.
I haven't been disapointed, i really enjoy the extra types of game play added, bombing run was ok to get off of DM, but ut2004 is worth it over ut2003.