T8 is plenty of height for a BP, though the 3" extra height in the T10 would be slightly easier for accessing the far back of the cage when cleaning and such. The belly heat from AP is kind of expensive. I just order heat tape from Reptile Basics and install under the cage myself to save some cash. As for the cage being too big for a young BP, you can order a pop in/pop out divider and just keep your critter in half of the cage until they're large enough to feel secure in the full enclosure.