You can use plex remotely (outside your wifi network at home) to stream your media. I think you can do that with airvideo also but it may take some manual router configuration to make it work.
But, plex's real advantage is the metadata details you get for your videos. Movies and tv shows are matched against internet databases by the plex media server. And, if you have home videos or more obscure unmatched commercial videos, you can enter the details yourself.
Plex also keeps track of watched/unwatched status and where you are in a tv series.