The main functional difference between these two shelters is that the X-Mid uses a wider rectangle (67" vs 42") so it can rotate the inner onto a diagonal. That layout gives much larger vestibules and eliminates the need for guylines because pole locations are no longer right along the edge. Space inside the inner tent is very similar in both tents but the X-Mid has much more vestibule space and a simper (4 stake) pitch. The High Route has the advantage of a very small footprint that can fit into small sites. Besides that there are some smaller differences, such as the X-Mid uses polyester fabric so it doesn't sag in the rain. Mostly I'd decide based on whether you prioritize vestibule space or a small footprint.
(Disclosure: I designed the X-Mid).
