The differences between orcs must die and dungeon defenders are...

OMD, you have more strategy because you can create traps to help you in waves, dungeon defenders is a co-op gameplay, that you just have to kill, you dont have traps (atleast I didnt see any, just saw a video), if you like more strategies you should go OMD, if you like more killing and that things, you should go dungeon defenders.

Thats my point of view.