In my experience living up North, block heater comes in first, and oil pan heater second.
Why? Block heater is heating up the cylinders and combustion chambers, which is where you want it to be warmer. That's going to get the combustion process going easier with warm surfaces and less metallurgical contraction.
Oil pan heater is great too, because you have warm, thinner oil circulating the engine.
Both together is definitely better, but the block heater is the better of the two in my opinion since it going to help the engine fire off faster.
I had both in my RX7 because I didn't want the engine drinking coolant. The rotary system for subzero starting was a separate coolant bottle that allowed the engine to drink a little coolant to help seal the apex seals. Mmmmmmmm.....no thanks.
I definitely wanted the oil pan heater, because cold starting that engine meant it automatically went to slamming right up to 3000 rpm and staying there as soon as it came on. This on an engine with 80psi hot oil pressure. Seemed to me that a true unassisted cold start probably had the oil looking like brown foam instantly.
Block heater was more necessary because metallurgical contraction had even more exaggerated results than a piston engine, making the apex seals nearly unable to seal. Great situation to end up with some serious FD and long crank time.
Had myself a battery heater and charger/maintainer wired in too, since I rarely drove it in winter.
