Either one will do the job.
The idea of these tiller locks is not really to provide an auto pilot I find that most of their usefulness comes from releiving you from tiller duty to go upfront to reef sails, drop anchor, raise/drop sails, go into the cabin to pee or grab a drink. When single handling the assistance is priceless. These are activities that are independent of the size of your lake. Don't expect precise steering from either, that is achieved by balancing the sail plan but they give you adequate time to do quick chores before the boat slowly veers off course.