Personally I have both and they are great..
as mentioned, they are different types of games.. if you want a fast paced blow em up game UT2K3 is the game for you.. if you want a slower paced game and the ability to fly planes, drive tanks n such BF1942 is super fun.. as mentioned, the Desert Combat mod is fantastic and adds a more current military overtone.. I started flying the Apache helicopter recently and all had a good laugh as I spun out of control and crashed into an enemy base.. once you get the hang of it, it's not so bad.. I managed to get in some kills before slamming into the ground.. the M1A1 tanks are fast and powerful..
anyhow, yes, they are different types of games and I personally recommend getting both of them so you can have a change of pace... I often switch between them...