Chiming in on an older thread.....
Putting on my engineering cap, the two types of heaters serve two different purposes. The coolant/block heaters primary function is to aid in starting & provide a quicker warm up. The oil pan heater is more to prevent excessive wear on the engine resulting from the oil flowing much slower due to the higher viscosity. The ideal situation is to have both. Having one over the other can actually cause excessive wear, and here's why: Coolant/block heater only could give a false sense of 'warm up' & the operator may then run the engine harder, before the oil has had a chance to warm up. No-the warmer coolant isn't going to warm the oil very quickly due to much less flow rate and volume. As for having only an oil pan heater, as mentioned in an earlier reply, the oil would cool down very quickly once its circulated in the cold block. BUT....its better than no heater at all. If I had to choose one over the other, and since I've never had a problem cold starting my Kubota in sub zero temperatures, I'd go for the oil pan heater-to reduce the excessive wear caused by cold startup.
Also-if your machine is hydrostatic, another heater mounted under the trans is a good thing for the same reasons.