The only vent free that would not have a bad effect on indoor air quality is electric. Otherwise, stay away from ventfree. The Consumer Reports website used to have a detailed explanation of why vent free is bad. Some building codes consider it a violation, especially in a sleeping room because of the safety hazard.

If you want real heat, and not just looks, consider a gas fired direct vent system. But they are costly. Also, the Obama administration is offering up to $8 thousand tax credit if you buy a fireplace that complies with certain energy specifications. I'm not sure of all the details, but they use either wood or "biomass" and are energy efficient and pollute very little. A tax credit is much better than a tax deduction, because it's subtracted from your actual final tax bill. So If you owe the IRS $20K, and you have a $8K credit, your net tax bill is $12K.

I imagine that fireplace dealers are aggressively promoting the units that qualify for the tax credit. They are worth looking into. Just make sure they don't jack up the prices and give you a smaller benefit from the credit.