Smartwool midweight / Patagonia capaline 4

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Daytime temp of 15F, I'd go with the R1, which is also great for sleeping in. My new R1 hoody was a perfect match for 15F when paired with an OR Ferrosi hoody. Can't say I've ever been more comfortable when hiking.
At 0 to -10F, if you sleep warm and are wearing down layers under a 10F quilt, I doubt the baselayer's going to matter much.

I have several 200wt Icebreaker tops that make great sleepwear when brought for that purpose, but don't think much of merino for active wear.

I like the Cap 4 for very high aerobic activities- as mentioned above- it simply dries quicker than wool; for less than very high aerobic activities I prefer wool- especially on multi-day outings. For sleeping I find wool much more comfortable and warm.

If it was a ski trip, I'd bring Cap 4, if it was a snowshoe trip, I'd bring wool (hooded Indie)