There are important ssafety hazards with vent free. And they are a violation in some building codes, especially if in a sleeping room. They can dangerously deplete the oxygen level in the room and can also produce combustion pollutants.

The best most energy efficient system uses Direct Vent technology, but they are costly to buy and sometimes complicated to install. We have one and love it.

An alternative gas log system that is vented and produces the most realistic flames that I have seen is made by Eiklor (?? the spelling). They will probably heat a room more than most vented gas log systems, but still send most of the heat up the chimney. Check their website.

If you can manage it, get direct vent. Stay away from vent free.