It's hard to generalize about sizes because the fit can be "performance" (snug) or "comfort" in the Duofold wicking shirts. Generally, though, I find that one size up works fine. If you wear men's M get women's L. For base layers you want a closer fit. I would rather have a snug fit than a loose one and Coolmax is very stretchy.

I don't know how they compare to Under Armour, but I do know that I personally am willing to compromise for the difference between $5 and $30. Duofold products are very high quality.