After extensive tests and comparison I found the Concerto viola bow produces a somewhat smoother sound than the Sonata. I think that has has to do with the slightly softer stick of the Concerto. I think this has also some effect on the modulation of the sound. Of the Sonata I especially like the even better "grip" which helps to unfold all the potential of my instrument without even getting close to the bow's physical limits.
Due to the very good balance of both bows in combination with the reduced weight, compared to wooden bows, I experience a completely new level in the ease of play and relaxation of the right hand. The Arcus bow and the right hand form a unified whole, and so you never have the feel of holding a strange thing in your hand. I decided to take the Sonata for now, but will probably also get me a Concerto some later day.