Bath exhaust fans are sold by name, grille style, and finish. The label is the part worth comparing. Start there. The airflow rating, the sound rating, the duct size the fan is built for, and the control type are the lines that decide how the room will feel once the work is done.
Airflow
Airflow is listed in cubic feet per minute. A rating only means something against the room it serves. Ask the seller or installer how the required airflow was worked out for your room. A common method uses the room's floor area and ceiling height. Write the answer down, and compare it to the fan's rating, not to the rating of the fan beside it on the shelf.
Noise
Sound ratings are measured on a stated scale at a stated distance. Only compare fans that list the same scale. A fan that looks quiet on one scale can look loud on another. If the fan will run during a shower or while someone is sleeping nearby, the sound rating is the line to read twice.
Duct size and run
A fan rated for one duct size will not deliver its rated airflow through a much smaller duct. A long run, a sharp bend, or a crushed section cuts airflow as well. Ask whether the installer will measure the duct run and the outlet. Check that the outlet has a damper so outside air does not come back in when the fan is off.
Controls
Fans are switched, timed, or triggered by humidity. Match the control to how the room is used. A timer helps a room that stays damp after use. A humidity control helps when the room is used at unpredictable times. A plain switch is simple and works as long as someone remembers to run it.
Paper to keep
Keep the model number, the receipt, and the warranty card together. Read what the warranty asks you to do to keep it in force, such as registration or a record of proper installation. Those are the steps that get skipped, and they are the ones you will need later.
A short comparison
Before you choose, write a line for each fan: the airflow rating and the room figure it is checked against, the sound rating and its scale, the duct size and the run it will use, the control type, and what the warranty asks you to keep. Fans that look close on the shelf often separate on that page.