Standing on the outside looking in, it seems the mortiser is both more economical and more versatile than the Domino. If you want to drill a larger diameter with the mortiser, you insert a larger diameter bit. With the Domino, you have to buy the larger machine. Ouch! With the mortiser you could make tenons, with a little improvising. And then there's being able to drill a lock mortise or any other variety of mortises. The Domino seems really more of a doweling system than mortising system.
The Domino 500 is $960 and the 700 is $1400. The Hammer mortiser is $890 and the mortising bit set (6, 8, 10, 12, 14mm) is around $60. Their 8, 10, 12, 14, 16mm slot-cutting set is $260. Both go up to 20mm in diameter. And it runs off a 4HP motor.
The downside is trying to joint a board with the mortiser attached appears difficult, at best. It's not portable or easy to stow. And you can't just pull it out cut a few holes and put it away easily.
Or so it all appears sitting here in the bleachers.
“Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain