Haven't seen Side Effects yet, but highly recommend SLP.
Lives up to the hype for sure. Great performances all around, De Niro kills it, push a really thoughtful storyline with really funny moments as well. Can't go wrong with that one, and you should see it before Oscars if you pay attention to them because it will win several imo.
Plus, the crowds shouldn't be as bad as they will for Side Effects since it's been out for months. That is pretty much always my deciding factor anyway