For me, mid-weight merino is typically too heavy to carry as sleep wear. Not to mention, it is only worn for sleeping, so the no-stink properties are not all that useful. Also, the synthetics would dry quicker when washing (if you are on a longer trip). Lastly, merino is typically more expensive than synthetic counter parts.
Using Montbell as an example, a mid-weight baselayer weighs:
Synthetic (Montbell Zeo-Line MW) - 5.1 oz
Merino (Montbell Super Merino MW) - 6.7 oz
Again, this is for mid-weight baselayers. If you are using lightweight baselayers, the weights are closer to the same. You would then just be deciding if you want no-stink, or faster drying.
All of my long sleeve sleep baselayers are mid-weight. If it is not cold enough for a mid-weight layer (freezing or below for me) then I just wear the lightest short sleeve top I can find, and some Ex-officio briefs.