The Touratech is pre-drilled (square holes) to accept a RAM ball. The ball mounts diagonally on the Touratech. This is handy when you cannot mount the Touratech directly on the bars because of a lack of space, bar-size, visibility, or whatever.
The RAM allows multiple bar-mounts, extensions, etc., etc. and has a lot more flexibility in positioning. A downside is that the GPS is more vulnerable to theft unless you also purchase a locking RAM mount knob. You may want to carry the GPS and mount with you when you are off the bike. Also, the GPS may be more exposed if you drop the bike.
A big advantage is you can transfer the GPS/Touratech/Ram stuff from bike-to-bike easily by simply putting RAM bar mounts on each bike.