Always-on fans remove treated air from your house and push it outside. This means either your furnace or air conditioner has to work harder to rep;lace the lost air, driving up your heating/cooling costs substantially

If you are going to use always-on, you will need to vent through a heat exchanger to recapture some of the heat or cool as it leaves the house.

The better choice, however is a timer switch that leaves the fan running for a set period even after you turn off the light.

You do not need to vent for more then 20-30 minutes unless the bath is gargantuan. The switches work with any fan, so you don't need to spend the extra money on fans that have built in timers. The switches are a lot cheaper than a heat exchanger, and less costly to install. If they fail, you need only replace the switch, not the whole fan.

Figure about $30-70.00 for the switch. We buy ours from Conservation Mart, but there are plenty of other choices.

Another option is a switch that senses humidity and leaves the fan on until humidity is reduced to a preset level. These switches are more expensive, but they pay for themselves in a year or two..

We have been using these switches in bath remodels for over 30 years -- starting back when they were very rare and hard to find. They work well. In all that time we have had just one failure and the company replaced it under its lifetime warranty immediately.

For more detailed information, read Inadequate Ventilation section of Bathroom Mistakes That Owners Regret.