They look so similar that I had to review a lot of videos to come to my conclusion that the differences are: 1. high crotch has arms higher than single leg. 2. high crotch has head outside his body while single leg has head against (inside) his body.

But otherwise, they look identical, except that high crotch looks harder to be sprawled on cause the arms, shoulder, head are higher than single leg. So, in any given situation, why would you choose to do single leg over high crotch? I am told that in high crotch, you clear his arm by moving his arm to inside (I am organizing what I learn while asking questions so that my book knowledge can be more systematic), or to down, or to up. Wouldn't I usually have to do that for single leg as well?

(My favorite is still ankle pick. Especially 2 versions where I push his shoulder with my palm, and where I have collar tie on him. It is just that having an alternative plan while executing an ankle pick gives my opponent an issue with reflex, and this guarantees a greater success with my ankle pick. Or I can go into high crotch or single leg.)