The resistance is controlled by shifting the bike's gears or adjusting the resistance units themselves to different levels of difficulty. Bicycle ergometers are essentially stationary bikes with large flywheels, and meters or computer displays that will estimate how many calories you have burned or watts of power you are generating. Each of the units can be incorporated into a successful winter training program. The answer depends on the type of training you want to emphasize. If your goal is to maintain general fitness, and improve bike handling skills or develop a smoother spin, rollers may be the best choice.
Rollers provide very little resistance, so training in zones one or two is the norm.
Handling skills and pedaling technique are fine tuned because the rider is unsupported. To get started, try setting the rollers up in a doorway or hall. This will enable you to learn to ride them without falling. When you lose your balance just lean on the door frame or wall, but watch the elbows. Start in a moderate gear with an 80-90 rpm cadence so that the gyroscopic effect of the wheels will help keep you upright, and keep pedaling.