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.