I watched the series first, then read the manga (only manga I've ever read), then watched Brotherhood. I honestly like them about the same, the original did a good job of adding twists, and honestly the emotion in the original is a lot better. More happens in Brotherhood in my opinion but that's how the author intended it to be. I suggest watching both if you're a fan, they don't disappoint and it's hard to guess what will happen next on either!
It's especially fun to be surprised at changes in Brotherhood after watching the original. So the other day I was in the school's library's manga section, and I noticed two different FMA manga series. Are there two manga series? Or is it just the multi-books-in-one thing?