There clearly is some individual preference at work here.
I've tried both thin/light merino wool and capilene, and of the two I'll use the lightest capilene I can get ("Capilene 1"). It dries faster when wet, I find it to be slightly more comfortable, and the thinnest capilene layer I can hike in even when it's quite warm out.

That said, for an upper layer I only carry a long sleeved capilene as something to be both second shirt and warmth layer on colder trips; I far prefer a button-up (all synthetic) shirt for the pockets, ability to unbutton, roll up sleeves, something that fits me 'loose' rather than skin-tight.
Brian Lewis