What Cab and Andy wrote makes sense. I suppose one might be better for some situations than others. I recall my friends in Wisconson telling me that dipstick ones didn't cut it out there.
Of the three you linked, I'd say the magnet or the adhesive is the way to go. Magnet is removable - I think that what my friends used before they had a garage. But the reason I didn't like the Moroso is that the lack of direct contact will prevent it fro having effecient transfer of heat.

FWIW a block heater came on our '69 Belvidere. I assume it was dealer install. My folks liked to ski and it was common up in ski country (Vermont) for lodges to have outlets for overnight parking.