The main reason to use S7 for a waki, IMHO, is price - that's a big hunk-o-steel, and if it's a little cheaper, I'm sure many dogs will be happy.
I will buy it in either steel, but would prefer 52100 as well. I do kinda like the idea of having sword options in all three steels, though - makes an outright comparison between the three that much easier (sort of like SoD vs Camp Tramp).