I own both merino and synthetic, and I wear both........but in different conditions/circumstances. Here's my take:
-Synthetic: It's comfy and comes in a variety of weights to suit your body type (do you run "hot" or "cold" typically?) and weather coldness. They are definitely insulating. It's sood if you run cold and aren't terribly sweaty when active. Can be pricey for what you get, depending on the brand (ie-UnderArmor vs no-name). Tends to stink quickly, can get static-y against body hair.
-Merino wool: Wicks sweat, reasonably insulating for lightweight, well insulating for mid/heavyweight. Definitely feels heavier in the mid-weight when compared to synthetics. Soft to the skin (not like traditional wool) and can be used to regulate body temperature in cold or mild weather. Highly resistant to odor, does not smell at all if you're out in the backcountry for a few days without a shower. The fit can be "snug" or "relaxed" depending on the brand and style. On that note, I've tried Icebreaker and Smartwool. Both are expensive but I'd give the definite advantage to Smartwool; they have better design details (particularly in critical areas at wrist, neck, and crotch), generally have better attention to sizing, and slightly more affordable because they tend to go on sale a bit more than Icebreaker. Come in various weights, I prefer the lightweight stuff.
Since I tend to run hot in the torso, I typically use merino if I'll be working harder on the slopes. If I'm out with my kids and taking it easy at the local hill and it's really cold out, I'll go with the synthetic because I won't be sweating and the synthetics are better insulating and feel more "snuggly" when I'm standing around. The merino is better suited when I'm on bigger slopes and will be charging and working hard and won't be cold at all, and will be sweating.
Different strokes, as it were. Both are good, but pros and cons to each.
My suggestion? Get the Merino for the lightweight stuff, and get the synthetic for the mid-weight stuff. Then you'll have the best of both worlds to suit variable conditions, and ultimately you'll find you may prefer them for different conditions as well.