I have a pair of Rab merino "bottoms" as well as some Aldi merino tops and North Ridge merino tops. All are superbly comfortable as well as warm when you need it and also cool when you need it. Merino works well in both situations. The Aldi stuff is a bit thinner, but I think that can work in its advantage.
I have some man made type base layers as well, but they don't have the same unwashed lifespan as the merino, but are equally as warm. Never thought to use them when it's warmer though...
D.