Those are two great books-I don't see Movie Comics 6 up for sale often at all, AND it's a cool early robot cover. That Wonderworld has to be the earliest pure horror cover. I paged thru the monthly newsstand on the Mike's Amazing website (which was fun, by the way, especially if you start with 1935 and watch how month by month the comics medium grew), and I found a few earlier with horror elements, but nothing that was a clear cut, definitive horror cover like that one.
Maybe Detective Comics 6 would be a contender if the view were shifted 180 degrees so we could see what that was casting the menacing shadow (it almost looks ape like to me, but even then, would that be horror?).
As a side note, I had to read up in the Twilight collection after seeing that slabbed Wonderworld, since I wasn't familiar with it and I'm starting to get interested in pedigrees. Awesome copy!