I have a D2 and DII and both are great. Omega itself used those names somewhat interchangeably so name itself really doesn't say much. The defining characteristic is, D2 has gears cut into the slanted vertical post and DII doesn't. What that does is the head goes up and down by a handle crank for D2 and you just push and pull for DII. D2 also has a thicker slanted post than DII.
I actually prefer DII for some reason.