I have garments of both material types that you mentioned. I definitely like the Capilene more than the MTS. The MTS does an O.K. job at wicking, I suppose, but the material is sorta "scratchy," if ya know what I mean. It doesn't feel that great against the skin. The Capilene, on the other hand, feels great against the skin, and does a much better job at wicking. My only complaint, so far, with the Capilene is that it tends to "pill," even when the garments are still fairly new. However, I would spend the extra cash and get Capilene instead of anymore new MTS garments.
Capilene seems to be the layering material of choice for a lot of people, but I'm curious how well REI MTS products would hold up in comparison. Is it worth spending the extra $$? Any opinions?