They are both wonderful resorts. Pop Century has over 2000 rooms and depending on where your room is, it can seem spread out a bit. The food court is very, very good and the shopping is nice. Take some time to look through the glass cases on the walls in the lobby. There are items of memoribilia for each of the decades that are in the "classic" years. Pop has its own buses, where sometimes the AS resorts have to share. I have stayed at both resorts and the busing situation is about the same.
I really enjoy AS Music also. I was worried about that this past March when an imprompto visit left us scrambling for accommodations. We ended up at the AS Music family suites and loved, loved, loved the resort. It has a great deal of mature landscaping (trees) that are quite close to the buildings so it had a real get-away resort feel. The food court and shopping were on par with Pop. They have pool activities during the day and into the evening and then weather permitting a nightly showing of a disney movie on a very large blow-up screen at the main guitar pool.
I think either way you really are going to have a great time. Now, if you decide to try to switch back, don't let go of your Pop ressie until you have the other one confirmed. Enjoy your trip