As stated above: if you're in it for the coasters, BGA in a heartbeat. Montu, Kumba, and SheiKra are all world class rides, and there are, depending on your tastes, at least a couple of other worthy coasters (Scorpion, Cheetah Chase, and, if you hit them on a good day, the Gwazis). The zoo aspect has been toned down a little in the last few years - no more alligators under coasters - but is still a great selling point if you like that sort of thing.
IoA is more of an all-around experience, though. Amazing theming, great variety of rides/shows, great food, and great rides, including two awesome water rides. It does tend to get incredibly crowded in the summer, though; much moreso than Busch Gardens, in my experience, and I really try to avoid Universal/IoA until the fall even though I live <2 hrs. away.
If you're planning a one-time visit to the area in the summer, I'd honestly lean towards BGA - lighter crowds, better coasters, more shade, free booze if you're legal...for a spring/fall trip, I'd probably suggest IoA, or better yet, a couple of days (at least) split between Universal/IoA/Sea World.