I'll take the cold over the rain any day. You can dress for it, even WAY below zero. Plus snow tends to just brush off instead of soaking you.

The rain gives problems with handling and visibility that you don't get as much of when it's cold. I also try to use my ears a lot to gauge the distance/speed of the cars coming up from behind me - they're a lot louder in the rain, and the echos off of the buildings tend to throw off any doppler shift.

At any rate, neither keeps me from commuting, but the rain can keep me from going out and riding just for the fun of it. Snow or cold doesn't stop that. Actually, snow sometimes encourages it... Freezing rain is the thing I REALLY try to avoid.