I prefer riding outside if possible, but you need to invest in clothing and lighting. Yes, it can get into the 20's here, the only thing that really takes the edge off for me is black ice.

having said that, when I was recovering from collarbone surgery, I made heavy use of the excellent www.trainerroad.com setup. It totally transformed trainer workouts for me. You need a $30 Garmin ANT+ USB stick and a speed /cadence sensor (you don't even need a Garmin computer). They've calibrated wheel speed to power for a whole host of trainers so you can use their extensive set of power based training programs. They also have some workouts that are synchronized with sufferfest videos. If you don't have much winter gear, that's also your best bang for the buck.