I agree w/Dodgeman. Ventilation to the outside is better also for health reasons. If anyone in the house has 'sensitive lungs', unvented fireplaces will irritate their lungs.
Also, if you can get a fireplace that uses outside air for combustion, then you've isolated the inside air. Plus, your insert / unit should have an additional capability to, either naturally or via fans/forced air, circulate the home's air 'around' the heat source for use as heat. One of the reasons a stove worked in the old days is because it radiated a lot of heat into the air around it. So, today, by forcing (or by convection) air around the heat source, you can take real advantage of the energy that's produced by the unit!
In the house I grew up in, we had a combination coal/wood stove in the living room. My parents simply added a floor vent above the stove which allowed heat to rise up to the 2nd floor, thus, circulating around there to heat the home. Granted, there wasn't anything 'forced' about that system but it goes to show you how much heat can be generated! (It was so hot downstairs that my Dad HAD to put in the vent to allow some of the heat to escape!)