A bit late in wading in, but..... I tried the HDF and the SDL on a MM3 60. I could not see any difference in optical performance. The Opticron rep who was with me indicated that you would only see a real difference if you compared the two eye pieces on a bigger scope. Both worked very well with the MM3 60.
I did raise the waterproof question, and was informed that if I dropped scope and eye piece in a pool of water, there should be no water ingress as long as the screw connection is snug.
I have now been using scope and HDF eyepiece for 6 months with no problem.