Thanks for posting this. I've spent countless hours overlaying topo and aerial trying to better understand my property's terrain features in relationship to food, bed, an travel corridors. This is very very helpful.
Also look at Caltopo. A good portion of the data on Hillmap is pulled directly from Caltopo, but instead of just a side by side you can also view in percent shading overlays. I kind of like seeing the aerial photo showing through the topo for certain applications.