From what I understand, the only difference between teh two, is that teh top bearing is metal balls in the 3004, and a plastic sleeve (bushing) in the 3003. I also understand that the 3004 is now replacing all the 3003 in teh standard flight packs and Tx packages; becomming 'the standard'. (Which replaced all teh 148s at one point...)

Personally, I don't really see / feel a difference, other than the 'Oooh bearings' factor. One is as good as another.

Use them where-ever you wish, just make sure they're balanced ( use two 3004s - one in each wing... instead of one 3003 and one 3004. Just in case there is an actual difference.. but doubtful)