This is a rather poor example. If you distribute the C# code above as a library, people can extend and introduce another shape, while they cannot do it easily with you F# code.
I did mention the Expression Problem[1] in my comment, and explicitly called out subclassing as a particular advantage of C# (the OP asked for criticism of F#).
Also, even though F# can also subclass, that facility is a bit weak since it can't define protected members.
