I've got to make the same decision but I've got 3 months. Here is what I'm deciding:
Stock is staggered 19's with performance tires.
Option 1: Keep the 19s for summer tires and buy 18's or non staggered 19s with winter tires.
Option 2: Find winter tires that fit the staggered 19s and upgrade to 20s and max performance summer tires.

Option 1 would be less $$$ and not wasting the current tires. But I want the 333M wheels.
Option 2 could be done in parts, 19 in. winter tires late November and 20's next spring.

__________________
Richard
Sir Snaps-a-lot, 9 Time Dragon Slayer (54 runs!)
2011 X5 50i Alpine White, Biege Nappa /Lt Poplar M Sport
2009 Z4 35i, Black Sapphire/Ivory-Black/Anthracite

Gone but not forgotten: '08 550i, '06 X3, '06 650i, '02 M5, '99 540i