If you are deciding between AKL & Poly and are planning on spending most of your time at MK, then I strongly recommend going with the Poly.
I completely disagree with those that disagree with the monorail being convenient to the MK and Epcot. Especially if you are at the Poly. Then you wouldn't have to transfer monorails to go to Epcot. And from the MK, you are only two monorail stops away.
Take a look at a map, look at where MK is, and then look at the location of the Poly, & AKL. It's a no brainer.
Also, if you can swing it, book a theme park view room, because I'm guessing that if you have an 18 month old with you, there won't be a lot of late nights in the parks. So, this gives the adults the chance to sit out on the balcony and watch the fireworks while the little one sleeps in the room.
Also, the rooms at Poly are quite a bit roomier than at AKL.