I voted Lego, but in full disclosure, we don't actually have any Jacks, which I suspect might be worse. I've never ever stepped on a Jack in bare feet though, including when I was a child (Yes, I had Jacks as a child. Yes, I stepped on toys as a child including Legos many times, but never ever Jacks.)
I think that some toys (Legos, Playmobils, woodentrain sets, little cars) are more likely to be stepped on regularly than other toys. Often the reason is obvious, one is unlikely to step on a balloon b/c it will bounce harmlessly away a bike b/c it stays outside. I don't know why Jacks have eluded being stepped on.