Whether you should get a block heater or a heating pad depends on what your aim is. A heating pad on the oil pan will heat only the oil (the heat that is transfered to the rest of the engine is very small), and that heat will be instantly soaked up by the rest of the cold engine when it starts. This of course doesn't mean that a heating pad alone is bad - any amount of heat that you put into the engine prior to startup is good - and a pad is easier to install.
On the other hand, a coolant heater (installed either on the coolant line, or directly in the block) will transfer heat to the most of the engine. This means that it might not go into rich mode when starting, saving fuel, not to mention the cleaner emissions. In my diesel the warm coolant also keeps the glow plugs from turning on, reducing battery drain in cold weather.
Of course, a coolant heater doesn't heat the oil down in the pan, so right after starting you may notice a temporary dip in engine temperature. The best solution is to heat both the oil and the coolant/block. I have a 550W DEFA coolant heater and a 125W Wolverine pad under the oil pan and am very happy with how they work together
The bottom line is: If possible install both, if not go for the block heater and thinner oil, but a lone pad heater is still better than nothing.