I only wear merino wool while hiking, no matter what the conditions are. I have a locker full of short/long sleeve, crew/zip neck, thick to thin.
I own several pieces from each of Icebreaker, Smartwool and, Ibex. I am satified with them all but, my critique would be the Icebreaker has the best feel, Ibex the most durable (hate the embroidered emblems, though) and, Smartwool just does it's job without standing out in any one area.
Personally, I would not go under 140 gm/m weight in any merino product. I think the smartwool microweight simply isn't durable enough as friends have had issues with that one.
And yes, why doesn't everyone use very light colors in their thin weight, short sleeve shirts! Aghhhhh! .... There, now I feel better.
And Miguel, stop trying to tempt me with those Mont-bell shirts! I have enough already! .... I think?