Another reason to skip MGM is that if you do not plan your times well you could miss out on a lot of the shows. They time them so you have to really rush from one to another to get them in. My husband is a very experienced Disney Addict and is very good at mapping out routes thru the park and he has a hard time planning the shows and such so we can see more than a couple in a day. Our last trip we saw the most ever and I think that was 3 plus all the rides. (And we were there in September, zero lines.)
My daughter (5) is afraid of the 3D shows. My son could care less about Beauty and the Beast, Little Mermaide and such. He likes Playhouse Disney because they blow bubbles for a bit. He loved pushing the buttons at Millionare, but really spent most of the day in the stroller while sister rode the big rides.
As for spending 2 days at MK, I would say plan 2 days for MK, if you decide you don't want to do 2 days at MK then throw in MGM. But when my kids go we spend at least 1.5 days at MK. We usually go in September when there are no lines so they are able to ride every ride they like 4-5 times, which is usually enough for them. If we were to go at a busier time 2 days would be the minimum we would go to MK.
This is how I would rate the parks for our family. 1. MK, we spend at least 1.5-2 days each trip, no matter how long or short. 2. AK, we didn't do this park until last year because everyone told us not to. My kids love it. My 3 yo (was 2 at the time) loves the water rapids. They have to be 38 inches for this one. That's his favorite ride, the safari, they have a dinosaur ride (think Dumbo with Dinosaurs), We couldn't get them out of the dino playland. They both loved the Tarzan show. 3. MGM (only because my 5 yo love TOT and Star Tours, otherwise it would be at the bottom) When my daughter was <40 inches this was definatly our last choice. 4. Epcot