There are many reasons NOT to choose arborvitae for bonsai: loose foliage that doesn't form neat pads with training, flexible branches that don't hold shape after removing wiring, etc.
However, I think that the loose foliage and the long shoot-like branching lend themselves nicely to a literati form. Also, they are very cheap conifers! hard to come by for many folks.
We'll see how it turns out!