what's the difference between Fullmetal Alchemist and Fullmetal Alchemist Brotherhood?

FMA anime was made before the manga was finished so it had differet stories and an anime only ending
FMA Brotherhood follows the manga story

which one should i go for?

I would recommend watching Brotherhood, it follows the manga more than the 2003 series

Both are excellent but brotherhood is slightly better

Both! Watch FMA anime first. Some people like Brotherhood better, but others like the original anime better. Both are really good, but I personally have a soft spot for the original anime (I find the tone more fitting for the themes and it is less shounen-y). However, some people say if you watch Brotherhood first it ruins the orginal's experience a bit, so watch FMA first, then Brotherhood! They're really different (beyond the first few episodes), so you aren't going to be watching the same show twice.