engine block heater or....
I'm a fan of engine block heaters in a freeze plug hole. First choice, however, is a battery blanket usually only 80W. If the battery is warm, it will provide the cranking amps to turn even a cold engine over, but if the battery is cold the capacity will be greatly reduced and may not even be able to crank a warm engine well enough. If a vehicle is not used for a few days in real cold weather, a three stage trickle charger about 1 to 1.5 A is the #2 item I would use after the battery blanket. Surprising how much the standby electronics suck capacity from the battery when it's really cold out. Last would be the engine block heater. I would rarely use a tank or radiator hose heater, they just don't trip my trigger as they provide for more leak points, IMHO. Engine rebuilds that I have done always get a block heater in a freeze plug hole. Heck, I've even put two in big blocks in the past during rebuilds. YMMV.
2002 Liberty KJ, 1999 Grand Cherokee WJ x2, 1963 404.0 Unimog