Very technically speaking, Brotherhood IS the original series. The writers for FMA obtained licensing rights from the creator of the Manga. The first several issues of the Manga were already in existence when they started work on the show and they were allowed to use the beginning of the story to start off. After that they were on their own.
So they began their own plot line which was completely independent from the comic. The show became a whole new entity and Hiromu Arakawa continued down her path with the book.
Now with Brotherhood, they are bringing the original story to the screen. If you noticed, the first 10, give or take, episodes of Brotherhood were kind of like a Cliff Notes version of the first. That was because we had already seen that portion of the story and there was no reason to go into all the detail again. That's what DVD collections are for, lol!
Now that the story has diverged, several episodes ago now, it has slowed back down to the kind of pace we were accustomed to with what we watched first. Only now it is going in a completely different, albeit just as intriguing direction.
Brotherhood must pull at the heartstrings more, because I catch my teenage daughter tearing up alot more with this one. The scene she has found the most endearing, and heartbreaking, is when Winry faces her parent's killer and hold Scar at gunpoint. She bawled! Personally, I just found myself thinking over and over, "Yeah! Do it! Do it! Blow him away!" but maybe that's just me, lol!