I use a 20H since I like the extra eye candy having more plants for a nice visual for the room. It doesn't take up much more space the a 10 gallon.
More room give you more space to work with and add extra plants. Although people say that tincs and azureus won't use the vertical dimension, if you build it, with decent access, they will venture wherever physically possible. I notice my azureus pair use the entire tank. They will climb the entire background all the way to the lid and sleep in the pile of dried moss, and use all the floor space for hunting. Also an adult azureus can be quite large for PDFs, so I felt I would rather err on the larger side of things, even though sucess can be had with smaller tanks.
Here is the scale of an adult azureus female and a 20H.
Funny thing is I gave my pumilio even more space than my azureus.