There are reasons to pick both, it just depends on which are important to you.

Positive Features of Vented Gas Logs:

Extremely realistic flame pattern that looks like a wood fire.
Flame is much taller than vent-free logs
Flame dances and wraps around logs like real wood.
Does not require a Carbon Monoxide (CO) detector
Since the damper is open, most of the smell will go up the chimney

Negative Features of Vented Gas Logs:

Less heat than vent-free logs (probably won’t heat your room)
Uses slightly more gas than vent-free logs

Positive Features of Vent-Free Gas Logs:

Much better heat output than vented gas logs
Less pollution
Slightly less gas consumption

Negative Features of Vent-Frees Gas Logs:

Since the byproduct of burning gas cleanly is water, vent-free logs will introduce excess moisture into your home which can result in mold or mildew.
You must be careful not to burn your vent-free logs for extended periods of time with windows closed, otherwise they will deplete the oxygen in your home.
If you burn your vent-free logs for more than an hour or so, you must crack open a window to allow air into your home to replace the oxygen burned by the vent-free gas logs. This will introduce cold air into the room and offset some of the heating benefits that vent-free logs claim to have.
Vent-free gas logs produce an odor that many people equate to the smell of burning kerosene. So if you have a sensitive nose, you may not be able to handle the smell and will be greatly dissatisfied with vent-free logs. On the other hand, we’ve heard some people who like the smell because it reminds them of their childhood days at Grandma’s house.
Vent-free logs are not legal in California as well as some cities and municipalities. Please check your local regulations. Please note that it is not illegal to use Vent-free log sets if they are used with the vent open.
Vent-free gas logs are required to have an ODS (oxygen depletion system) as well as a Carbon Monoxide (CO) detector.
The flame pattern with vent-free logs pails in comparison to vented logs. Because the gas must burn cleanly, the flame height is very low and most of it is blue. Also, you will find that the flame does not dance around like a real wood fire and very little of the flame actually touches the logs. The reason for this is that when the flame touches the logs, it gets cooled off and does not burn cleanly and starts to produce soot and exhaust gases.

Ken – http://answers.yahoo.com/activity?show=cb639d7423d550eea3a368fd9f0aaa52aa