I had both the Ue-10 PRO full-soft and hard acrylic materials. To my ears, it is a matter of personal choice: I firmly believe that the level of comfort is derived from the quality of the ear impressions. The full-soft material has the advantage of a slightly fuller fit 'n feel in both of my ears and slightly more sound isolation (though I must adamantly warn members that I have no scientific facts nor tools with which to come to such an absolute conclusion). Furthermore, the full-soft material is designed for professional musicians because the softer acrylic material is more pliable and flexible; hence, professional musicians who need to contort their faces while singing and dancing on stage prefer it. As to the sound difference, the full-soft material sounds rounder and softer. However, this comes at a cost: I found the full-soft material to be less durable and I was responsible for causing accidental nicks and abrasions to the full-soft material while I used the wax loop end of the double tipped cleaning tool. After discussing this phenomenon with Ms. Harvey, she recommended the hard acrylic version because I am not a professional musician. I took her advice. I now have the hard acrylic version of the Ue-10 PRO. The biggest difference that I noticed is that the entire surface of both earpieces are smooth to the touch and harder. As a result, I have no problems using both my double tipped cleaning tool or Tech-Care Clean-Wipes on the surface of the hard acrylic earpieces. There are no nicks, cuts, or abrasions whatsoever. The other big difference is that the hard acrylic material is a better fit in both of my ears. In other words, the hard acrylic material feels less full in my outer and inner ears. As a result, it feels like a more contoured fit and it is much easier to slip in and out of both my ears. In fact, I can do so with only one hand in under one second flat. I also do not experience any pressure points exerted on any part of both my ears. The biggest sonic differences are that the hard acrylic material sounds even more extremely accurate in terms of sharper precision, greater details, and a fuller sound stage. As to why, I simply do not know as I mean only to report my findings.
Ultimate Ears has decided it prudent to recommend the hard acrylic material for the non-professional musician market and the full-soft material to the performing artist. I concur with their wise decision. The greater durability of the hard acrylic material has the distinct advantage of more longevity and lower total cost of ownership because audiophile headphone owners need not resent their UE products back to the company for repairs or maintenance under normal real-world applications. Furthermore, it should be noted that there are perceived sonic differences between both the hard acrylic and full-soft material as mentioned in the above paragraph. Lastly, comfort is the direct result of the quality of your audiologist's ear impressions and I find the hard acrylic material to be equally comfortable as the full-soft material but it possesses the distinct advantages of far greater durability, longevity, and easier maintenance.
I want to make my findings public because there is a definitive trend toward purchasing custom IEMs within the audiophile headphone circles. To that end, I want to present an articulated, balanced, and objective statement. Please feel free to contact me with private messages or replies within this thread. Thank you very much.