I personally would in NO WAY recommend a ventless fireplace or heater. They produce gallons of water vapor under the best of cases and noxious fumes as well. They also produce CO which could lead to death as you well know. Yes they have an oxygen depletion sensor on them, but I wouldn't want to risk my life on a sensor that could easily fail. Great Information on it here
So, definetly go direct vent, they are safe and have proven the test of time.