I prefer option a) for the constructor form, and b) for the quantification
syntax. I'm strongly sure that the explicit quantification is a much better
choice, while I think the first choice (constructor form) is more of a
matter of taste.

I think the constructor form using "of" is better as the general function
syntax suggests currying is possible (| Foo : a -> b -> c), while it is not
possible for constructors (or you maybe it is, and then should support it).
"of foo * bar * baz" is not perfect (I prefer the revised syntax "of foo and
bar and baz"), but at least it's a consistent defect we're used to.