What's the difference between Picture (Windows Metafile) and Picture
(Enhanced Metafile) in the Paste Special options? Is there a good rule as to
when to use one over the other?

At a quick look for pasting Excel sheets, they seem the same, but I'd guess
that's just because Excel sheets don't have any fancy graphics, gradients,
etc. But that's just my guess. Perhaps Windows Metafile is fine for basic
shapes and Enhanced Metafile is better for advanced vector images?

Thanks,
Colin