In a onepiece mount if a screw on the base gets loose the base will move.On a two piece base if a screw works loose on a bases the rings will help hold the scope in place more so than a single base.And for the money as well weaver are good bases,and they have IMPROVED rings now. Not like the old ones that were mentioned,plus they have a nice finish. the new rings have a dual lock system on them and no problem in setting up your scope as in the past.It nice to keep up with whats on the market. So go with weaver.The new rings are called "Quad Lock" hope i helped. vangunsmith