A heatpump will always be more efficient than an electric resistance heater. But, as outdoor temperature decrease, a heatpump's output diminishes. An electric heater is 100% efficient in that there is no waste - all of the electricity used is turned into heat. A heatpump is greater than 100% efficient - a heatpump can use X energy and extract/transfer 2, 3, 4 times the equivalent heat, if that makes sense.
But, like I said, the capacity of heatpump decreases as outdoor temps decrease. For example, a 3 ton heatpump may produce 36,000 BTU at 50*F, but may only produce 20,000 BTU at 25*F. This effect is compounded due to the fact that your house is also losing heat quicker when it's colder outside, so the heatpump has to work harder (longer).
In my house, we have a 5 ton and a 2.5 ton heat pumps. The 5 ton has a propane furnace and the 2.5 ton has heat strips as backup or emergency heat. The 5 ton does a much better job at producing heat in its zone as the temps drop, but even then, I switch over to propane when the temp drops below 30. The propane heat is hotter and is more comfortable to my family. The 2.5 heatpump struggles below 40, so that's where I switch it over to the heat strips. Again, the strips produce a hotter heat and is more comfortable. At the end of the day, the heatpumps are more efficient when it comes down to $$$.