Since Newmar has had a history of upgrading its models every couple of years to the standard of the next higher model. I would guess that the 2017-18 Dutch Star would be pretty equivalent to the 2014-2015 Mountain Air. And, as @Neal said, newer is usually a better choice (less miles, etc.).
I think what would be most important in choosing between them are things like floor plan, preferred chassis, options, etc.
2017+ for the improved star chassis, hydraulic full wall slide, and overall improved design.2016 should be avoided for dutchstar. Even with a replaced axle, the coach has very little carry capacity and has many small bugs in design.
2015 is considered a sweet spot for pre star foundation coaches. It has raised roof lines, and the original fws design. Plenty of carry capacity, and very robust reliability.