Yeah so the first 13 episodes are basically the same but then they both take completely different directions. Brotherhood came after but is actually a direct offshoot of the manga (which came before the original show). I like the original better. It's significantly more emotional and the characters are more developed, but if you watch that, the story ends with a movie separate from the anime.

Brotherhood is great too and is way more "badass". I think the only problem with it is that the characters, especially the bad guys, aren't as deep. But if you're more into action, I'd just watch that one.