I was always taught that a single is easier to get, but a high-c is easier to finish. I'm a freestyler myself but I imagine the same holds true for folkstyle.
It all depends on who is shooting and who is defending. If you are a "machine" at either technique, it won't matter.
If you shoot hard enough it doesn't matter, I like one leg takedowns myself, but as long as you are committed and follow through you can't go wrong.
My coach believes the high crotch is. I think his thinking is that an outside step makes it take a long time to get to the leg and once you are in on a high crotch the finish is so much quicker and easier to complete.