I would definetly go with AK, and I think probably MGM. I think that older children get more out of Epcot..there is not much for a baby there. And at 18 months, your baby will be so into the characters and music and excitement...have fun!
I also say AK and MGM. We brought my daughter when she was 15 mos. old and she wanted to do quite a bit. That's a nice age, before fear sets in, so they may be willing to do more than they would at 2-3 yrs. I was wondering, are you ruling out MK?