The only reason to buy a motor of lesser thrust is cost. If you have the funds, get the most powerful one you can afford. You can always run the powerful motors slower, but when conditions such as wind or current overcome a lesser motor, you won't be able to control the boat. Having a stonger motor will get you to the next spot faster when pattern fishing rather than taking to time to pull up the trolling motor, starting the gas motor to move several yards (dock fishing is one example). The first thing I did when I purchased a new 16' aluminum bass boat is remove the supplied 12V bow mount motor and install a 36V motor. I dont' feel like I have overpowered my boat. I agree with Olebiker about the variable speed models.