I LOVE Mustang in both...
In FMA, he was quite a bit different as he had a different role but I liked it.
But in FMA brotherhood.... (Apart from the change in seiyuu) He is so smoking hot and awesome!!!!
Episode 19 of Brotherhood is all the reason you need to love Mustang more in that Anime, as many others have already pointed out. I had a nerdgasm once I saw him, well, I guess I shouldn't spoil it for anyone that hasn't seen Brotherhood yet. He definately gets way more epic moments in the second Anime
I personally like him in both but if i had to choose i'd have to say in Brotherhood.Let me explain why:In FMA:Brotherhood Roy showed alot more emotion when Hughes died.When he fought Envy (and Lust , as well) he went crazy!He totally kicked ass and it showed JUST how mad he was about his Best Friend's death. And it showed a little bit more about his relationship with Riza.(As in their trust and the whole 'Shoot me if i lose my way' thingy.
But Roy is always awesome so yeah xD I just think Brotherhood has got lot more emotion so that's why i like him (a bit) more in part II
He's great in both ! If I had to choose I'd say in the first FMA ! In the manga his hair and forehead are different . He's great in Brotherhood too , but I like first Roy more ! And I'm really angry with what happened with him at the end of the first series !!!!
In my opinion, Roy is stronger in character, more sympathetic, and it shows how he depended on his friends/ subornates in fmab, but in fma it showed a more clever side of him. In both Roy showed leadership, passion, and dedication. Overall i prefer Fmab over Fma.