I think the Deltapoint Pro is more geared for range and competition use, and the RMR is more for duty and EDC use. For competition, I'd prefer the bigger Deltapoint Pro, but I go with RMRs preferably or Holosuns for carry.
The rumor is, the Army has selected the Deltapoint Pro for their M-17/M-18. The Deltapoint Pro seems plenty rugged to me, but I would agree, the toughest one of the bunch would be the RMR, I'm confident you could run it over with a truck and it won't break. I have a suspicion the Army selected the Deltapoint Pro because it has the biggest window. And when I was shopping for my first RDS, I thought the biggest factor for performance would be the size of the window. I have learned since, after getting acquainted with shooting with several different RDS's. The average sized window is all you need, bigger windows may be nice, but not worth the down side of the size creating all the problems.
When I shot on a cold rainy day, I noticed my Deltapoint Pro fogged up while my RMR did NOT. But, that might be because the Deltapoint Pro's lens was dirtier than the RMR.
The Vortex and other sub-$200 RDS, I can see the argument they are less rugged and more geared to range use. It does appear they are able to keep the price down by foregoing all the extra's for the rare exception condition and be less rugged. And that cold rainy day, my Vortex was the first to fog and fogged the worst of the three.
The Deltapoint Pro's huge window, and thus size is its biggest downside, the space creates more problems with holstering, drawing, catching on things while handling. And the other part of that size, I'm pretty sure, the larger window results in a larger base (the whole thing works off geometry and angles with the window slant and curve and the LED in the base) and the Deltapoint Pro's base is too large and creates problems with co-witness sights. There are no sights made for most pistols that will be tall enough to co-witness with the Deltapoint pro. The Deltapoint Pro has provisions to mount a rear sight on the back of it, and Leupold sells a rear sight you can mount on it, but no front sight. My M-18, I have tried two different front sights with a rear aperture, the best I can do with the improvise iron sights with the Deltapoint Pro is 3" high from POA.