displacements

Specifically mountainous terrain. I use the sculpting tool, pushing to be specific, but when I try to emulate natural-looking terrain, it just ends up looking blocky and awkward. Not to mention that it is incredibly time consuming, only to end up with a product that looks like it belongs in an...

In terms of rendering and performance, is it "cheaper" to make the floor of a map out of brushes that can seal the map on their own, or out of displacements which require nodraw brushes underneath them to seal the map? Does the increased file size from using the second method matter much?