Patch, VentAxia are the market leaders , the dogs ******, are known for quality, but tend to be the most expensive . If the duct is only the thickness of the wall then an axial fan will be OK .
If there is 300 miles of ducting then a centrifugal is needed.
I just fitted the Airflow Icon in my bathroom . It has an iris shutter on the inside to stop backdraught , if there nothing on the telly you can sit and watch it open !! As said above though , it is circular.
Go for the VA .