You can't scrub them like glass, but you don't really need to either. Gunk doesn't stick to them the same way. Usually all it takes is a gallon of lukewarm water, a tablespoon of PBW, and some shaking. If it's really gross then I'll stick a soft rag in there too, and with the rag they're always spick and span within a minute or so.
Soaking works too, but I prefer the shaking because a) I'd rather just get it done and b) I'm a teensy bit worried that excessive exposure to caustic chemicals would shorten the life of the plastic.
As far as relative merits, to me it comes down to this: Both kinds require lots of careful treatment. The difference is in why they do. With plastic, if you aren't careful you might need a new carboy. With glass, if you aren't careful you might need a new carboy and a new hand or foot.