In choosing, there is also a practical consideration.
illinoensis is a tall plant, leptolobus grows almost flat against the ground.
That would help define where you could or couldnt grow either and what you could grow them with, as well as functions.
ill. could be a summer time screen for a chain link fence.
lepto. could be a lawn! No joke, if people grow ornamental strawberries as lawns why not leptolobus.
Say your in a colder climate and want cactus, get lots of identical sturdy black plastic pots. Plant half in the ground empty and plant leptolobus around them, plant cacti in the remaining pots, put the cacti pots in the ground pots in spring, any water and ferts that piss out the bottom of the cactus pots would feed the deep roots of the leptos- thus cutting down on surface watering and thus weeds, in the fall bring the cactus pots indoors and put decorative stepping stones or something on the ground pots until spring.
Be creative.
Its true lepto can be hard to buy, just ask around the ethno community. More and more people are growing them.