My thought is that if you spend a lot of time on the winter commuter, go as decent as your budget allows. You probably have an instinct for what amount of money you could lose to a theft and still be OK with it, deep down.
There is probably some number at which those two considerations meet (overall budget & the ceiling on what you could really stomach losing).
Maybe there is some decent quality, used bike that fits the bill.
"The best rides are the ones where you bite off much more than you can chew, and live through it." ~ Doug Bradbury