Question

What’s the difference, if any?

Answer 1

None.

File.Open() is, internally, nothing more than:

public static FileStream Open(string path, FileMode mode, FileAccess access, FileShare share)
{
    return new FileStream(path, mode, access, share);
}

If you don’t use the overload which specifies a FileAccess and FileShare, it specifies this for you (using FileShare.None, and FileAccess.Write on append or ReadWrite otherwise).

That being said, this is an implementation detail, not part of the documentation. Technically, a future .NET Framework release could use a different implementation, although I find that unlikely.