I have watched both FMA and the old one is more memorable than the new one. Of coarse, I prefer the old FMA than FMA Brotherhood because the old feel more like it was made for older teen audience.
I haven't finished brotherhood yet, but I much prefer the original anime. If I had watched brotherhood before the original anime, I'd have been so confused D:
Plus the hair lineart on Ed annoys me in brotherhood xD
I've watched FMA (2003), and I'm still currently watching Brotherhood, but I honestly can't decide atm. They're both really good, and I really did like Wrath from the 2003 one, so I can't decide hm. The one thing that bothered me a lot was how they changed Rose's colouring, that was really pointless? And I got used to what she looked like before so it's like um, what.
I watched both over the Summer, and I must say, it is my all time FAVORITE anime. My suggestion, I watched the regular first, and watched Brotherhood second. I liked the regular better.
I think they're both worth watching if you really enjoy the series.. just Brotherhood follows the manga more closely so there's a lot added story-wise and some of the characters are just way different than in the original anime series, I've yet to finish Brotherhood myself but it's one that I plan on coming back to before long! :3