How are elliptical winglets better than blended winglets?
The best way to understand why elliptical winglets may be better than blended winglets is to read the text of the patent obtained by the inventor of the elliptical type, Dr. Fort Felker, a former NASA wind-tunnel researcher and expert in the field of computational fluid dynamics, and co-founder of Winglet Technology LLC.

The short answer is that the curvature of the winglets as they extend outward from the intersection with the wings approximates part of an ellipse (hence their name), and this apparently ensures an elliptical distribution of lift both horizontally and vertically, resulting in lower induced drag than a blended winglet.