Hmmm..well, we LOVE AK, but we really do NOT like MGM for some reason. We took our DD there last year to see Bear in the big blue house and she cried the whole time. Besides the really great 3D movie, I can't really remember anything else we did. We only spent half a day there and that was more than enough.
AK on the other hand was just amazing. We love the nature walks where you can view all of the different animals. We saw the bird show which was great. We absolutely love the 3D movie, "It's tough to be a bug" (Believe it or not, DD was NOT afraid of that at all!). The safari ride was so much fun, we ended up doing it twice. Most people will tell you you only need half a day at AK, but for us it is a whole day trip because there is so much to do and see for us. My DH's theory as to why some people may not like AK as much as the other parks is that it is more of a Zoo rather than a DW park that has a lot of Disney Magic. Where we live we don't have Zoos so for us it is a great change of scenery and a more relaxing park day compared to Epcot or MK.
Good luck in deciding....