I have seen a lot on here about the Blizzaks being "marshmellowy". I put a set of them on my Son's M3 because of the Chicago weather and for the fact that I didn't have to worry about him getting caught on the Interstate in a snow storm. I installed a set of the LM22 Blizzaks on his E46 M3.
Those tires were absolutely AWESOME. Best holding tire I have ever seen on a rear wheel drive car. I never noticed any "Marshmellowy" feel to the car at all, and I am a very spirited driver.
I just today tried to buy another set of Blizzaks for his E90 but it has Run-flat tires and I could not find the correct size in the run-flat option. I wound up buying a set of the Michelin Pilot A/S Plus ZP 245/40-18 Run-Flats. It appears that snow tires with the run-flat option are practically non-existent. You can forget about it with the 19s.
Another thing is about using the "square" set-up. What Ben says is true... the narrower tire will tend to hold better because it typically digs to the bottom whereas the wider tire will "balloon" and sit on top of the snow. The only problem with doing that is if you have the staggered set up, it tends to "stretch" the rear tire and makes it resemble a rice-burner. That is also why the "Model T" Ford had no problem even using a street tire, it was very narrow and would dig right into the most rigid part of the surface. The difference between the 18" and the 19" wheel is basically because it is easier to find a snow tire for the 18. There is some difference in the ride, but for the most part, you won't notice the difference in traction if both wheels have the same tire.
One MAJOR difference is the factory BMW wheels are forged and most of the after markets are not. Out of two sets of the after market wheels (8 wheels in all) I had to buy 7 extra wheels to replace the bent ones
. Only one time do I remember hitting anything. It was a pot hole that should not have bent the wheel, but it did
. When your wheels feel like they need to be balanced or cause the steering wheel to shake or wobble, check to see if the wheels are bent. If you install a set of after-market wheels, try to get a set of forged wheels that won't bend. You don't save anything by buying a cheap wheel. My .02 cents worth.