the soloist short shank is WAY better than the long one but for reasons unrelated to the long shank. The shank size is just a cosmetic thing that doesn't effect the sound but just helps you identify the era the mouthpiece was made. Its like when we say 2-line or no-usa link. its just a cosmetic feature to help you know what the internal and facing features might be like.
the long shank soloists i have seen (i have not looked at that many) had the strangest short facing cut like a scooped circle into the rails. the short shank soloist has a normal morgan type facing and a better shaped chamber. the long shank has a little too much material in the floor and a little baffle that makes it sound awful compared to the shorter one.