Literarychic,

What type of tread pattern do your tires have now? You may not need a winter-specific tire at all, and certainly studded tires would be overkill for all but 2% of the riding conditions we see around here, and have significant penalties for the other 98%. I'd probably advise against them, in fact.

Riding in snowy conditions is not that hard-- and we scarcely get snow anymore-- but since we use salt so heavily anyway, the roads are usually clear within hours of a snowfall, and mostly just slushy. Most of the winter, the roads are dry. You may encounter icy conditions on neighborhood streets for a day or two after a snow, but generally the treacherous spots are few. A little practice, good technique, and little bit of tread will get you through some frightful looking conditions pretty easily; the inexperienced are quite surprised by what cyclists seem to ride through, but the reality is that it's just not as difficult as they think it would be.

I've had great success with cyclocross tires for Ann Arbor winters, and am pretty delighted with Kenda Kwicks in particular, now on my second set and probably 5th winter with those tires. I wouldn't necessarily recommend them for you-- a little more tread would probably give a little more sure-footedness in the worst conditions-- but I mention them to underscore the fact that one just doesn't need a big, heavy, gnarly tire to ride the winter. In a winter tire, I prioritize fast rolling in the dry, and low weight (I don't want to strain when I'm hoisting the bike to hang it in the garage; I've got enough junk on my bike already!) as much as I do traction.

The short of it is, having ridden virtually every winter in Ann Arbor since '87-- I had a five year interlude while at MSU-- it's pretty easy to get around town on the roads here, and even the sidewalks, particularly around campus and on designated bike route sidewalks, are clear the bulk of the time and don't really require anything too particular in the tire department. A little tread'll do ya!

EDIT: Oh yeah, if you can afford a second wheel set, that really changes the equations a bit, and reduces the downsides to selecting winter tires for the worst conditions.