have extensions to go out of the roof and others just disapate in the attic area.
I would look for a good unit that would not rust, made from rust proof workings and one that has maybe a three way switch for speeds on the fan motor.
Make sure when its installed that the unit is positioned in the attic so that it does not wet or contaminate any insulation up there. Usually there is a outlet hole for vapor to exit. This outlet must be pointed up so that it does not just shoot on adjoining glass.
You might want to affix a flex dryer cord to the outlet and drape it over a rafter or support so that it dries faster.
The problem with most fans/lights is that they rust up over time and that the fan motor is too weak.