YOU ABSOLUTLY WANT ONE PIECE MOUNT.If you go with 2 piece, and one comes loose, your scope wil more than likely move. With a a piece, if you lose a screw, your 1 peice steel base is still pretty solid. might move some, but less than the 2 piece will.after that the rest is just preferance, as far as height...etc. I suggest rings by Badger ordiance, but they are $$$$$$$. I think around $200 or so. Might bhave gone up in the last couple years. They are big, steel rings that have been made specifically to match the other half of the ring. They are a bit heavy too.
All the other brands here are goreat products too. For me, my preferance is the Badger Rings.

Good luck to you.