Moog Audio is awesome -- got to try out a whole bunch of stuff when I visited Montreal this summer.
I own an Orbit and have tried the Mastotron, I would say there is no comparison. There is one setting I liked on the Mastotron and almost all of the knobs are all the way up or down -- totally at the extremes of what it can do, so you can't tweak anything once you get to the synthy sound. On the other hand, it's cheaper, it does that one gated synth tone really well, and it's pretty easy to use.
But that one really good setting at the extreme of the Mastotron is where the Orbit starts (FM side completely off, int/gate at about noon). You can get way way way more synth sounds out of the Orbit and it functions well as a noisemaker (if you put the mix towards the FM side) and doom fuzz (if you open the up the gate). A little pricier, and not intuitive to use, but it is completely worth the money and time.
If you're not convinced, watch the John Davis Tonefactor review for the Orbit.