Anything that both animes do, the original anime does better.
There are things in the original that do not get screen time (or get very limited screen time) in FMAB that they expect you to know about.
After episode 14, FMA diverges from the comic's plot massively and becomes weird.
FMAB is an adaptation of the complete comic.
They are both worth watching. Each having their own charms, but if you're only going to watch one, watch FMAB
I watched fma before reading the manga to any extent then fmab came out and I preferred fmab. after watching fmab I rewatching fma I would always think "ugh that's now how it was originally supposed to be"..
so I would suggested fma first to prevent that
I say watch 2003 first, and then Brotherhood.
That's how me and my boyfriend did it, and it felt far more rewarding that way. I feel 2003 got us used to the characters and gave us a great connection to them, and then when we got to Brotherhood, it was the same, as if the story never really ended. We think both of them are still amazing. Just start with 2003 first though.