F# coding style - static vs instance methods

I've noticed that static methods seem more popular in F# than C#. In C#, you'd always add an element to a collection by calling an instance method:

mySet.Add(elem);

But in F# there is usually an equivalent static method:

mySet.Add(elem)
// OR
Set.Add elem mySet

And I've seen the latter form quite often in samples. Both appear to me completely identical both semantically and functionally, but coming from a C# background, using the instance method feels more natural. Which is better style in F# and why?