1500R and 1800R basically defines the curvature of the screen.
The lower the R rating, the more curved the screen is. Basically, the idea here is that the higher the curvature the more it would follow the natural curvature of the a human eye which is 1000R.
You must note that a lower R (higher curvature) does not mean a better experience. In fact, a mere 24 or 27 inch FHD monitor with a 1000R curvature would offer a fairly strange and perhaps a distasteful experience.
The most common and recommended curvature to go for is 1800R. It offers both comfort as well as a great cinematic experience.
A deeper curvature, like 1500R, makes more sense if you are serious about building a multiple monitor setup.