If you have 50mm fenders I would imagine either size would fit. The tread blocks are not huge, and I think are counted in the tire size. I think the tire is measured including the tread blocks. The studs stick out like a milimeter extra.

35mm tires cut through fresh snow to the road surface a little better than thicker tires, and roll a little faster. The 35mm schwalbe marathon is what I use on my bike and what I recommend for people who ride on well plowed roads and plowed bike trails who have serious ice over the winter but otherwise want to go as fast as possible (I live in Minnesota).

The wider 40mm version is likely better at handling ruts caused by ice on the road, while not sacrificing to much in terms of speed or cutting through fresh snow to the road surface. However, if you're more concerned with grip on the road, I would think the 45nrth Gravdal (38c) would be a better choice than the 40mm marathon. It's a tad slower, but also a tad better on ice and dealing with snow - from the reviews I've read.

As a bit of a side note, in my experience there only reason to get above about a 40c tire for winter riding is because you have to ride crappy unplowed snow and can't afford a fat bike and it's a cheaper compromise. 30c-40c the tire cuts through light snow to the pavement below it. 2.0" the tire floats around unsteadily halfway in it which is kinda the worst of both worlds. At fatbike tire sizes the tire floats on top of it well enough to be an advantage, far far better than the 2" tire. In my limited experience at least.