Well, best is going to be subjective, isn't it? All a matter of personal preference.
I've stayed at both All Stars and Pop Century. The room layout at All Stars was nicer for us. We didn't feel quite as crowded. The rooms seemed better insulated against outside noise as well, although we didn't really have troublesome noise issues at PC.
We far preferred the Pool Bars, and Food Courts of the All Stars to the bar/lounge area and food courts of Pop Century.
Walking around Pop Century is nice though. We like to walk.
Parking at Pop Century wasn't the hassle for us that it has been for others, so that wasn't an issue for us.
Bus service from both seemed the same to us.
I didn't care for the cobblestone walkways of Pop Century when taking out rolling luggage to our room. I thought that if I'd been in a WC it would have been a really bumpy ride!
That's about all I can think of.
I'd go with whichever resort had the best rate for the time I wanted to stay.