I think it really depends on what you're looking for. Personally, I think MGM is a better park as it gives more of that "Disney" feeling. It has some neat thrill rides...such as Tower or Terror which is really quite amazing with it's attention to detail. When you first walk into the park, you are immediately greeted with a feeling of excitement. The thoroughfares are wide and look very sharp and eye-appealing. Also, they have some nice restaurants there with much variety.
I never really became a big fan of AK as it just seems less entertaining...and it doesn't offer as much dining options as MGM.
So, I guess you have to decide what you're in the mood for...want some glitz with some tinseltown flair or do you prefer a jungle-like, very hot animal atmosphere? I say hot because this park really feels like it's the hottest park out of all of Disney.
Whatever you do...have yourself a great time!