So, I set the tank up tonight and used a Fluval E-series 50-watt I happened to have. First time I've used this heater: I love the display, the precise control, and the plastic cage to protect the heater.
On the other hand, this thing is flipping huge for a 10g tank! To have it upright, I had to dig a pit in the gravel so the tip wasn't buried. Even so, the top is still almost at the surface.
And, because of the cage, this heater takes up a lot of space in such a small tank. Seems to me this is better suited to a sump, where it would be out of the way.