out of the lot...the Mesa Tactical Aluminum ones are very nice and seem a little more refined than others. The next best is the Tacstar Sidesaddle. It would appear to be the most common by far and best bang for the buck probably.
couple of catches though. When you get it, take the screws out that mount the plastic shell holders to the aluminum back plate then replace them with low hold Loctite. They do work loose with recoil over time and the screws are a pain to look for.
the other catch is that if you take the thing off a lot to clean then you will eventually wear out the threads on the mounting plate. Tacstar warranties these though.
Boltgun