Really, it comes down to what you can afford and what activities you do. Synthetics are highly durable, high-wicking, and have some odor-beating technology. Synthetics usually have a lower price tag.
Merino is less durable, but it has temperature-regulating features that can work in a wide range of weather, combats odors, and also wicks well. Merino wool typically costs more than synthetic blends.
More specifically, Merino is often woven with other fibers for longevity, elasticity, and fit. The percentage of merino varies in each design, which is why some wool blends are warmer than others. Be sure to check the percentage of wool to get a better idea. The Black Diamond Women’s Solution 150 Merino Baselayer Crew is approximately three-quarters wool and one-quarter polyester, while the Patagonia Women’s Capilene Air Crew and Bottoms are basically a 50-50 blend.
You might notice we left silk off this list. Silk needs a lot of washing, is very thin, and is not very durable. Most of the women’s base layers on this list are a wool blend or polyester-elastane blend.
The weight of the fabric you choose is also important. Some people will do well with a pair of simple lightweight base layers. If you’re perpetually cold or doing a sedentary winter activity, grab a pair of midweight or heavyweight base layers or a wool blend.
If you’re buying base layers to backcountry ski, run in, or for any other intense activity, go light. Easy peasy.