The circular one is a twist-loc that prevents it from disconnecting accidentally. The -|| 20A design is great in theory, but you only want to install a recepta

The circular one is a twist-loc that prevents it from disconnecting accidentally. The -|| 20A design is great in theory, but you only want to install a receptacle of that type if it is the only one on the circuit (and I mean only receptacle, so not even a single duplex) because what would be the point if you can plug in two devices that both require 20A. I've put these next to a window to power an air conditioner. The problem with the twist-loc is that the 15A and 20A designs are not compatible. You can't plug a 15A device into a 20A outlet, but 15A twist-loc receptacles are rare. So it is not uncommon to put a 20A plug on a device that draws much less than 15A if say you don't want someone to unplug your router by accident.