I would take a windshirt.
If it really is below 25, snow will be dry enough to slide off a smooth windshirt and you might be able to leave the rainjacket at home. If you are out on a longer trip or the weather looks like it might be damp, bring both the rain and windshell.
20F, headwind, sunny:
If you hike in your base plus fleece you still freeze in front from the wind, yet you boil in the back.
If you hike in your base plus rain jacket you sweat to much, leaving you chilled later.
If you hike in your base plus windshirt you stay comfy in the front, and only sweat under your pack.
With synthetic base layers and a windshirt, I would leave the sleap shirt behind, you can always sleep in your fleece if your baselayer is damp.
What about sunglasses?
So you are hiking in just trail runners and sealskins? No gaiters?
I find a windstoipper hat like the Dome to warm to hike in anything but the coldest and windiest weahter, also it's annoying because it muffles sound. With that in mind, are you comfortable hiking in the balaclava when it's warmer than about 15F?
I would second the wool socks for sleeping over the sealskinz, you want to give your feet time to dry out sometime in the 24hrs.