The next day I whipped up some wax frames (I'll post photos for those who don't know how to do it) and went to the hive. Sure enough, Al was right. The one hive that had supers with wax foundations were busy storing honey. The other with the plastic foundation was hardly touched.
Now they sell plenty of plastic - and I'm sure lots of people have good experiences with it - but I'm with Al--and so are my bees :)