block heater is best IMO, to get the engine up to a reasonable temp so it will fire, (I do not have glow plugs), but getting the block up to about 32F helps starting the tractor a lot, a block heater will heat the coolant and the head will get warm, also the filters are on the block and they get a little warmer than ambient temperatures,
the tank heaters work, but not as well IMO, I have one tractor with a block heater, and the other is a little older and no place for a block heater, thus the tank heater, I tried a propane fueled tank heater but unless I have a good wind break it will blow out, (wanted some thing that I would not need power for)
there is a "trick" I have yet to try, but one puts quick connect couplings on your gas truck heater hoses, and a similar to the tank heater hook up on the tractor but with out the tank heater but here the tank heater would go a second set of quick connects, and one usually can get a gas engine started easer than a diesel when cold, you warm up the truck and then hook the truck into the tractors cooling system via the quick connects, and pump the warm coolant through the tractor engine warming it up, thus being able to start this, (To my under standing the oil pumpers use this to get there pump engines started when it cold out), and I get many heavy equipment operators do the same.
I feed cattle and nearly regardless of the weather I need to get a tractor started to move bales with,
there are also oil pan heaters to warm up the oil, my JD tractor has a plug one can remove and install a factory unit, thinker or warm oil less friction, and better for the engine lubrication,
the faster one can get the tractor turning over the easer it will start,
one more thing having a trickle charger on the battery can help wonders as well . the charger will keep the battery at near full capacity,
Cold temperature increases the internal resistance and lowers the capacity. Batteries that would provide 100 percent capacity at 27°C (80°F) will typically deliver only 50 percent at –18°C (0°F). The capacity decrease is momentary and the level of decline is related to the battery chemistry.
I did this on my truck when I lived in Montana and it would get to -40F and the tickle charger would do nearly as much good as the heater as the engine would turn over much faster, and have done it on my tractor as well, here in Colorado,
after starting let it idle for a time and let the oils warm up some,