I have never stayed at either, and I haven't even gotten out of the vehicle and walked the grounds at either. So I can't vouch for bus service, food court quality, or anything else. So all I can do is give my personal preference regarding theme alone. Based on theme, if I had to choose between one or the other, it would definitely be Pop Century.
But after posting this, I just realized that All Star Music does have one thing that Pop doesn't, if I am not mistaken. And that is the family suites. Having stayed at moderates, with a room size of 314 square feet, the rooms at the values are 260 square feet, I believe. After this year's trip, we will be traveling with two children, so the small rooms at the values wouldn't cut it. So with that in mind, if we were to have to pick between those two resorts, I would probably pick All Star Music, simply so we could have a family suite.