Both versions pencilled by Jack Kirby and inked by Dick Ayers.
My guess is that after finishing the first version, Stan looked at it and said,
"Gee, this is too cluttered and you have to read the balloons to figure out
what's going on." So they simplified. It's also more dynamic
not to have all the figures in a scene be the same size.
It's odd though that
they didn't add "All New" anywhere on the cover. One of the things that generally differentiated Marvel
covers of this period from the concurrent DC books was that DC annuals were all
reprint, whereas Marvel's were all new. (Well, of course.
Fantastic Four had only been around for two years. If they were going
to do a Fantastic Four Annual, what were they going to reprint?)
Another possible selling point they omitted can be seen on the unused cover
where it says, "The Only Unbroken 37-page Comic Mag Tale Ever Printed!" It
wasn't that but it was an uncommonly long (for the time) story. Stan also
dialed down the hyperbole a bit. The original cover touts "The Origin of
the Fantastic Four" as "The Most Requested Saga Ever Printed!" and also says the
lead story contains "The Most Eagerly Awaited Battle in All of Comicdom!"
As revised, only the former claim is made. This is an example of Stan