A single arm full motion TV mount gives more flexibility as to the possible viewing angle you can get. In theory a single arm bracket can be turned right around 180° back on itself (the size of the TV will dictate how far you can go). The one downfall of a single arm bracket is that they are prone to sagging or twisting making it difficult to have the TV level when both fully in or fully extended. If the TV is going to be left out at a viewing angle (e.g. across a corner) then this is not an issue as any sag or twist can be allowed for during the setup.
A dual arm TV mount has two arms making it a much more stable option when the TV is moved in and out on a regular basis. A dual arm TV bracket is typically easier to install and to keep the screen level no matter whether it is fully pushed in or fully pulled out from the wall. The two arms will restrict how much of a viewing angle you can get with most only allowing the TV to get to about 80°.