They are both terrible, and "prime" Alex Smith is an oxymoron if I've ever heard one. The big difference in my view, is that Heinicke is being paid what he's worth, and didn't cost draft picks nor players to acquire.
It's hard to get too worked up with a below average to average QB when they cost so little. Also, I have to admit that I do like that Heinicke will at least take some risks when down. Alex Smith would check it down regardless, aside from that one season in KC when Tyreek Hill was getting 5-10 yards of seperation on damn near every play.