When I ask this question I don't mean between fullmetal alchemist and fullmetal brotherhood. I mean witch do you watch as a series.
Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
Both series are in fact separate entities.
The series named simply Fullmetal Alchemist was created first and went in a very different direction from the source material once the basic premise was established. Brotherhood instead closely adapts the original manga in it's entirety.
Edward and Alphonse lost their respective parts trying to resurrect their mother, Scar and the Homunculi exist and both series start off in Lior but that's where the similarities end. Major details like the origins of the Homunculi and the overarching plot are entirely different.