Slicks for 95% of the year. You can ride with slick on snow if you are caught out and they grip just as well as most other tyres in the wet when you are on tarmac.
If you know it is going to snow then some cx tyres would see you right. The studded tyres are heavy and expensive. Might be worth having in reserve but I wouldn’t out them on unless you are expecting issues. Some xc race tyres would also be good for the mtb. Quick enough yet providing grip. I assume you aren’t going to be looking for anything technical so you just want a bit more security on roads and tarmac?
I commute year round. Mostly on slicks. I take my mountain bike if I know it will be bad. Mainly because it has a lower gear and is more stable if the conditions are bad.
The only time I really come unstuck is when it is icy but then I just try and modify my route a bit. Most roads are gritted and fine. There are a few dodgy small ones but when it is snowing you can often get a better ride heading off the well trodden routes as fresh snow is good for grip.