I have a set of 18" wheels for my 535d GT that I bought last year with winter tyres after trying it in the snow with summers on 20" the previous winter and getting stuck pulling away from a junction on a non-existent hill (luckily the car side stepped into a curb to give me enough traction to pull away). The winter tyres on the 18"s were a premium brand but I was surprised at how bad they were outside of the one day last year I drove and they were actually useful when it was -7c. They were quite old so binned them when the wheels came off the car.
If you still have two sets of wheels do not fit all season tyres, go for decent winter tyres instead. All seasons are a compromise you don't need as you've got two sets of wheels so can have the best tyres for the season all year round.