This is an old post, but I found it via Google - I'm shopping for dryers and had the same question about Vented versus Condenser.
The unregistered poster is wrong about vented being cheaper to operate.
Ignoring gas dryers and focusing on electric: Condensing dryers considered alone consume slightly more energy than vented in most cases apples to apples. (2 to 10 % increase in the condensing model from what info I've discovred - matching features.) Not a big difference, but still based on the sticker printed on the back of the units, the condensers seem more expensive to operate at first glance.
However!
If you put the dryer in a living space (most of us do) then you must consider the nice comfortable heated or air-conditioned air that vented dryers will SUCK out of your home and blow into the great out doors. This is a considerable amount of air. For every cubic meter of air blown out of your house, outdoor air will find it's way back into your home through any crack or opening it can find. No house is a space ship.
When you factor in this loss of indoor climate control energy the condenser units always beat the vented units. Depending on where you live, you could be facing -very large- "real world" energy cost difference between vented and condensing models. I saw a graph in one study that indicated that Norwegians enjoy a real world energy savings of 90% (yes NINETY percent) with condensing clothes dryers versus vented. Exact numbers asside, the general theory makes sense to me.
This Norwegian effect is probably because colder regions will lose more expensive heated air than temperate regions with a vented dryer. In fact the condensing dryers even add a small amount of heat to the room which suplements your heating bill during the winter.
If you live in a warm region, the savings are less but still real. In the tropics a vented dryer would be pumping your nice air conditioned dry indoor air out into the muggy hot outdoors.
On beautiful warm and tempreate days when all your windows are open, it would be an even wash. A dead heat. (Puns intended.)
I live in north Germany which can get very cold. I'll be buying a Condensing model to help heat my basement.
I'll just drain the condensed water to the same place my washer empties out.
Thanks to everyone for your posts!