Heaters... lower radiator, magnetic or glue-on oil pan, in-line tank, air preheat or a good old heated storage area all have their pros and cons. If it's not pratical to store your tractor in a heated area then I'd go with the lower radiator in-line heater. It's one of the easier to install, but not as easy as the exterior oil pan type. By warming the water jacket, and thus the whole engine eventually, they have less tendency to create condensation on the interior of the engine. The ultimate is the in-line tank type heater and probably the most efficient. The least efficient and possibly harmful, depending on who does the research, is the dip stick type heater.
All the opinions listed here are important and helpful, but same as mine, I would just consider them to be opinions. Research them all and you'll be happy with what you decide on.
Me, I have a heated storage area that seldom goes below 60F.