Trapper was great. BJ just seemed to have that certain, well, swagger and sense of humor about him. Trapper seemed a bit more "all-business" to me. (Caveat: It's been a long time since I watched MASH).
Prefered Trapper. By the time BJ was on, many of the scripts became personal political statements by Alda and Farrell, which I thought got redundant and pushy