Well, overnet will be an open protocol when it comes to the 1.0 version (looong time from now I guess, since overnet developers seem to like the idea of not having competition :-///)
Any way, overnet is exactly the same as Kademlia (except that Kademlia is a completly open protocol), so preference is Kademlia, no real need to import overnet atm.
Greetings!