If you're going to have a Hopper ticket (a wise investment, IMO), it will serve you well on a day that includes the Animal Kingdom. Unless you plan to go to AK on a night that has the Extra Magic Hour, the park will close at 5pm. That's a great opportunity to grab some dinner and then finish the night at a park that is open later.
Honestly, unless you really want to see all of the shows or the late-day parade, you probably will be done at AK before 5pm. Festival of the Lion King is the only must-see show. Tarzan Rocks is good, too, but only if you like loud rock music. In early May, it certainly won't take you 8 hours to see those two shows and catch all of the rides in the park.
If you haven't been to MGM before, and want to do all of the quality attractions in the park, it could easily take more than a day. Off the top of my head, I'd skip Sounds Dangerous, the Animation Tour and Playhouse Disney. But all of the other attractions are quite good, including the new stunt show that should be open in time for your trip. Unless you're willing to sacrifice some of the attractions, I really don't think you could do everything at MGM in one day.