At that difference I'd probably try to do it with thinset because it simply isn't that hard. Also, its much harder to deal with the mosaic being thicker than the field tile....versus all the examples in this thread. But even then, there is always a solution.
And outlets are the worst part about doing backsplashes. Without them they would always be the easiest jobs in the world, but because of them they can be a real pain. Some mosaics have variable heights in the actual pieces, and these just dont look good around outlets because you end up with gaps. Still, no matter what, there is always a solution to this stuff. Yours seems to be relatively simple compared to what it could be.