Workout Length
The length of your workout can dictate whether or not the session is best accomplished indoors or outside.
Pro cold-weather ride: Most cyclists can easily spend time on an indoor trainer for 30 to 60 minutes and achieve a quality workout. Fewer cyclists can spend two to three hours, or more, on an indoor trainer and achieve a quality workout. If your workout is over an hour, and you have friends to ride with, head outside. Individuals in the group can take turns in the front to minimize the time anyone has to spend facing the wind. Small breaks in the shelter of the group makes a big difference in the comfort and quality of the ride.
Con cold-weather ride: If it will take you more time to get dressed than it does to accomplish the workout, stay inside.
Current and Predicted Weather Conditions
Take a look at the hourly weather forecast for your area.
Pro cold-weather ride: Many cyclists would prefer to ride outside. If you can get your ride accomplished before the weather gets worse or a storm moves in, more power to you.
Con cold-weather ride: When the weather is predicted to turn from bad to worse and there is chance that you would be putting yourself at significant risk by heading outside, head for the rollers or the trainer. Live to ride another day.
Personal Preferences
If you love the activity, there is a greater likelihood that you will exercise. If your workouts become repetitive, boring, tedious or downright not fun, more than likely you will find yourself skipping more and more sessions.
Pro cold-weather ride: If you would rather pile on 100 pounds of clothes than to face another indoor training session, ride outside. The athletes that prefer to be outside know what to wear when riding in cold conditions. Most of them find riding in the cold to be more comfortable than indoor trainer sessions.
Con cold-weather ride: Certain cyclists would rather ride an indoor trainer for two to three hours than to ride outside in the cold. These cyclists will do multiple spin classes, hours on a CompuTrainer or recruit friends to ride on trainers in their basement for hours on end -- anything to get a ride in and avoid riding in the cold.
Just Ride
To successfully survive the cold, whatever you define as "cold", winter and emerge in spring with reasonable cycling fitness, do whatever it takes to stay active. Don't allow excuses to steal your fitness.
Go outside or stay indoors. In any case, just be sure to ride.