100wt fleece midlayer or medium 280gsm merino ??

Take the fleece. Midweight wool absorbs a lot of moisture and takes a long time to dry. Fleece works MUCH better as a midlayer. I've only found wool to feel warmer when used as a baselayer (not sure why).

I personally prefer a fleece vest as a cold weather midlayer. I've found a full fleece midlayer to be too warm to hike in when coupled with a windshirt, and too cold, without.

fleece … itll dry quicker with body heat

last thing you want is "warm when wet" in below freezing … cause unless it dries when yr moving … itll be "frozen when wet" when you stop moving …

think of all that "warm when wetness" when you put yr down jackets over at stops … that moisture needs to travel OUTWARDS … through yr down and shells …

+1 merino base layer, fleece mid layer.

Look into grid type fleece, lighter, absorbs even less water and dries faster.

And maybe a down sweater for breaks, camp and sleeping.

I also wear a synthetic vest when at -15 C. The fleece and merino alone is fine for the arms, but additional core insulation may be wanted.

Of course a wind shell is mandatory with fleece.

Take the fleece. I find that the heavier merino pieces are not nearly as warm for the same total garment weight. A nice microfleece zip tee is a great all-around layer for general three season hiking.