need help with bolt action scope mount

Most people here are looking at long range setups, so go with a 20 MOA or so mount. This cants the scope down to allow more elevation travel for longer range shots.

All the 20 MOA mounts I have seen are one piece.
 
Either will work fine. I do use a 1 piece base whenever one is available for a given action. Most of the time you will see 2 separate rings used from the base to the scope. This is for a couple reasons. First you can get a set of high quality rings for about 1/3 the cost of a single piece mount, and second a pair of rings generally allows for a wider range of adjustment to match your eye relief. Also the further apart you can set the rings for a given application the better your scope tube will be supported.