As my only car in a climate with a real winter, I'd seriously consider the A5 - damned good looking car and it has the ability to get you where you need to go in winter. The 128i would handle better for 9-10 months of the year, but could be problematic if you want to get to the mountains to ski / board, etc. I think you're also right that the Audi will hold value better than the 128i.
I have a full winter tire set-up on my 135i, but we also have an S4 Avant that's our primary winter commuter vehicle and is used for road trips through the Rockies in the winter.
Good luck with your choice!
2011 135i M Sport | Dinan Stage 3 Suspension | Dinan High Performance Bushing Kit | M3 Wishbones | Modified CDV | Clutch Stop | APEX EC-7 Wheels w/ Michelin PSS 235/40R18 265/35R18 Tires | Maddad Street Series Exhaust | BMW Performance Kidney Grills | JBSPEED Painted Spoiler | 28% Tint