Biggest difference in autohelms is the job that they are designed to do.
These categories are:
long term continual use
short term (i.e. abt 24 hours max)
the short term versions tend to operate directly on the wheel
or the tiller, and a very much weather
The longer term versions tend to work directly to the rudder
, and use either an electric linear drive
or a hydraulic unit for movement of the rudder
. The longer term units also tend to be sited primarily in dry compartments.
Some of the longer term units not only have the standard heading sensor, but also have rate gyros to sense speed of course alteration, and thus are able to better maintain the course, as they not only sense the change earlier, but also are able to apply an appropriate rudder correction due to speed of change.
Most modern units of both types will not only allow a fixed course to be selected, but also a feed from a plotter to a waypoint. The more expensive units also allow a feed from a wind direction sensor to allow courses in accordance with the wind rather than a fixed direction.
The bigger and more capable the unit, the higher the power requirements, especially for the electric linear drive