Haven't seen all of FMAB or FMA (never could get into it, thought the anime was really well done and executed from what I saw but the shonen aspects of it were just too much for me). From what I have been told though FMAB is suppose to be the 'better' version which had lead to many a great debate, one of the biggest reasons for FMAB was like Baudin said adding in the manga ending which is a much happier ending than than the original. Most fans of FMA watch both just because its more of the anime to watch FMAB has like 15 more episodes or something, and most people that like the anime REALLY like it lol.