The 18s would be better in snow if more narrow. they dig deeper for more grip.

As far as MPG, a harder narrower tire (all season, etc) would get slightly better MPG but not a big difference.

FYI, you need a larger diameter tire to improve MPG, not smaller. The larger tire reduces RPMs for a given speed.

__________________
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