I think a lot of people have seen these pics, but here is my setup from last year:
Hartmann HR8-GS:M 19x8.5 et25
Pirelli Scorpion Winter 255/50/19
These wheels weren't my first choice in the style department but Achtuning had a scratch n dent set (no visible flaws) for $880 so I went with it. I decided on 255 width tires for looks as I think even the 255s look too narrow on the SQ5, but the Pirellis have a round sidewall so they bulge a bit.
Traction is great in wet and snow, ride comfort is excellent, and they are still sporty for a winter tire. I'm sure snow/ice traction could be better, but it's still quite good. I've personally sworn off Blizzaks because they compromise too much dry/wet performance handling to maximize ice/snow. That's just the Blizzak way which works for people that see more of those conditions and don't mind less performance. The best winter tire I've driven is Michelin PA4 but no dice in our size there.
If I had to do it again, I would spend a little more and get 20x9 Hartmann peelers with the same tires (20" obviously).
There is no way I would fit 18s even if they did fit, but the vast majority will not fit up front.