x2
I glued mine inside the tank, over the 'floor' glass, with the UTH under said glass.
Why? - Mainly because of what our friend luvmyballs said, I like the T-stat to regulate the temperature OVER the glass, which will be a more closer reading to what the animal will feel (of course, subtract a few degrees due to the substrate). Also as mentioned before in the thread, placing the probe between the UTH and the glass WILL create an air bubble which will affect the area heated and the temperature as well.
But I'm OCD, and I hot glued that probe to a certain way that it's not entirely easy to be pulled even by me without applying a few Newtons (sorry, physics overload).