I've used a Kats 50 watt magnetic heater. It didn't do much at all. The only part of the heater that comes into contact with the oil pan is the two thin strips of magnet, and there's a sizable air gap between the steel base and the oil pan. Pad heaters are a lot more efficient. If you don't have room for one big pad heater, you could install two smaller ones.
Block heaters are much better than oil pan heaters for engine starting, even at the same wattage. The temperature of the oil in the sump doesn't matter at all for engine starting. It's the temperature of the bearings and the cylinder walls that will affect cranking friction. A block heater will also heat the cylinders, pistons, fuel injectors, and intake air, which makes for better fuel vaporization and combustion.
Some heat from an oil pan heater will make its way to the top of the engine, but not much. If I recall correctly, the 50W magnetic heater only increased the coolant temperature by a few degrees, vs 40°C with a 400 W block heater.
If the poor starting is due to low cranking speed, a heated battery blanket could make a bigger difference than an oil pan heater. A warm battery will not only increase cranking speed, but it will also charge faster once the engine is running and will more easily maintain a full charge, which can be a problem in cold weather if you're doing short trips. A battery blanket will use a lot less power, ~80 W, and will probably be cheaper.
If the poor starting is mostly because the engine is in a poor state of tune, heating the block will be most effective, but a powerful enough oil pan heater might make enough of a difference. Of course, you could install both a battery blanket and an oil pan heater.
