Synthetic baselayers dry quicker than merino wool, so this is what I prefer when fell running. However, merino has excellent anti-smell properties which make these garments much more socially acceptable. An alternative point of view is to utilise a synthetics quick drying time and wash it occasionally. Or you may want to use them in combination for varying conditions. For a great in depth analysis of their differing properties please see this link to BackpackingLight.