I really like (the nice parts) of Glasgow...and it's a very short train ride from Edinburgh: I would do as a day trip...but that would definitely eat into the very limited time you have in Edinburgh. In terms of the 'unique' aspects of Glasgow (and I speak as a non-Scot who has only been a few times), I particularly liked the Rennie Macintosh buildings/ artwork, and the rather quirky but charming Museum of religion (or whatever it's called).
BUT, taking up a whole day in Glasgow versus a whirlwind drive around the (accessible) highlands...would take the latter, even in Winter...
tb