I don't know the Battle Valor rules, but 40mm is the 'universal' frontage in 15mm (it's used in DBA, Hordes of the Things, Arte de la Guerre, Mighty Armies, Mayhem and many other systems). So, for the extra 10mm, you'd be keeping your figures compatible with a whole host of other games. In most of these games, depth is relatively unimportant or doesn't matter at all. Also, many of those games are designed for a small space. For example, the standard table size for Hordes of the Things in 15mm is 2' by 2'.
(Apologies if you know all this - just in case it helps!)
I've got a few Battle Valor miniatures; they're a bit bigger than some but well within the normal 15/18mm 'tolerance'.
All things considered, though, I'd recommend sticking with the standard 40mm: it unlocks a whole load of games and will make your elements that bit less fiddly to move.