What ever heating source is used, remember that having adequate circulating fans near the ceiling is as important for both uniform heating and conservation, is any decision about the specific kind of heating device. Without fans, the vast majority of warm air will be in the top 2-3 feet of the green house. The air movement provided by the fans will also help to grow more hearty plants and will help avoid conditions that promote various diseases.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.