Bathroom Ventilation- It used to be that a bath with a window did not require a ventilation fan. That is no longer true. Today, any room with a toilet, shower or tub must have an exhaust fan and it must duct to the exterior of the home- not into the attic.
The primary purpose of the exhaust fan is to lower moisture levels- not odors.
The best fans use a squirrel cage blower rather than the propeller type fan. They move more air at quieter noise levels (sones). For most baths look for a fan rated at 80cfm at 1.2 or lower sones level.
Kitchen Ventilation- There is no code requirement for an exhaust fan in kitchens. Depending on your cooking habits a vent hood may or may not be necessary.
Recirculating hoods use a charcoal filter to remove odors but do nothing for moisture levels.
Like bath vents, the squirrel cage fan moves more air at less noise level. If you install a vented hood it is critical to install the appropriate ductwork and be certain it is ducted to the exterior of your home.
Determining appropriate cfm's of air movement depends on your cooking habits and position of the hood. An island hood will require greater air movement than one mounted against a wall.