Budget for a "commuter" would typically not support a Ti frame, and steel would not be the best choice on wet and salty roads. Carbon fiber would be ok, but the price premium would be better spent on other things (see below). That leaves aluminum as the obvious choice. Other key aspects would be disc brakes (preferably cable actuated), fenders, and clearance for aggressive tires. Wheels should be sturdy and not too expensive so you can own two sets. That way you can quickly swap between studded and non-studded tires depending on road conditions. Some thought should be given to a low-maintenance drivetrain. An internally-geared hub with belt drive is probably ideal, but also pricey. At the least, invest in a stainless steel chain. Finally, don't skimp on lighting. Motorists are not looking for cyclists during winter months (if they ever are), but they'll pay attention if you're lit up like a snow plow.