We've stayed at Pop Century quite a few times now, and we've never paid for the preferred. Our first time staying there, were were in the 60s bldg, overlooking the kiddie pool, but it didn't cost extra at that time. We've also been in the 50s, 70s, and I believe it was the 80s. In the 80s bldg, we just cut through by the parking lot and we would be right by the bus stops.
We stayed at Pop for two weeks in July - had room 0101, first floor, first room right next to the Big Wheel. My brother & family met us later during the week and they had a preferred room. They were a few rooms over from us, on the other side of the Big Wheel. To me (and to them), we didn't feel it was worth the extra money.
Good luck with whatever you decide!!!! Have a great trip!!!