Pop. Queen-sized beds versus full size beds, plus better Disney IP theming that the kiddies will like. Now, not all of Pop is themed for the kiddies, but you've got better chances than at Music, IMHO.Advice: Go Youtubing. There are a lot of room and property tours of all of Disney's properties. Be sure to look at only the most recent room tours from Pop Century. Our first trip with the kid was at age 6. We stayed at Movies, in the Toy Story section, and the kid definitely loved the theming.
We stayed at Pop a few years back, before the renovation, and I liked it better than Movies in many ways. It felt newer and fresher. At Pop you're right across from AoA, and you can use the buses there if you need, both going and coming back. The bus service was pretty good, though we also mostly used our car to get around. We found that using a car to get around to the theme parks was almost always faster than buses, with one important exception: The Magic Kingdom. With the other 3 theme parks and the water parks, you drive, park, and then you either walk (water parks), or walk or hop a tram (Animal Kingdom, Hollywood Studios and Epcot), and you're right there. The buses might take you a little closer, but you're about at the same point as the buses and have freedom, plus you can park right outside your building, if the parking spaces favor you. MK adds the layer of the TTC, and the need to take either the boats or the monorail to the MK. Buses arrive right outside the MK, skipping the TTC trip layer, both ways. It can sometimes be faster in a car, but most times it's probably faster to take the bus from your resort to the MK. However, the boat ride and the monorail are both fun "rides" which have a coolness factor that lots love.
All that said, there's nothing wrong with Music. The pool qualities are about the same. I did prefer Pop's food court over Movies, though I thought there was nothing wrong with Movies. Watch some videos to try to get a feel of both resorts and go where your heart and pocket book tell you.