If your goal is to stop heat from escaping your home through the damper or the flue while the fireplace is dormant you are WAY overshooting by getting glass doors or an insert. That would be like buying a new cadillac to cover a grease spot on the driveway. There are more cost effective and efficient ways to stop your damper from releasing heat or letting in cold draft. You could use a chimney balloon damper to seal up the flue or a chimney top damper.
Now, if your hope is to get more heat from your fireplace when you are burning a fire… you are comparing apples to rotten oranges. A wood burning insert will allow you to get far more usefull heat from the fireplace (apple). The glass doors (rotten orange) will ensure you get next to no usable heat from your fireplace by blocking the little bit of radiant heat it casts forward. But glass doors are good for keeping smoke from entering the living area and for controlling the amount of heated air the fireplace sucks in.
fireplace insert will give more heat