In terms of LD50 figures, O. innesi has stronger venom, but considering the amounts that a Parabuthus can inject, I'm inclined to believe a sting from the Ortochirus would be less harmful. In my opinion though, we must always treat each specie with equal respect and precaution. For me personally, an Androctonus gives no more risk to its keeper than a Hottentotta when you keep this rule in mind.
Though, don't take a highly venomous spp. if you don't feel ready for it, this is what causes mistakes and accidents. Somebody on this forum thought me once not to fear them, but to respect them. Best advice I've ever got.
If the choice was mine, I would probably go for one of the Parabuthuses. P. tranvaalicus is overly massive, but leiosoma has awesome colouring.
If your planning on going for all of them at some moment anyway, just go for the best deal.