My 37' has.....
a Robertson hydraulic autohelm that I've come to love. My previous sailboat, an Irwin 31, had an ST4000 which served me well for several years. The 4000 is a wheel mounted unit with a separate control panel; the hydraulics of the Robertson are all mounted below deck, as is the electronic brain. Mounted on the exterior anywhere you chose, is the electronic control which is very user friendly. The performance difference is that the Robertson seems to work more smoothly than the 4000, which had a tendency to jerk as it steered. Of course there is a huge unit price difference and required installation skill level for the Robertson is much more higher. If price is the question, you'll find the 4000 much more affordable than the Robertson hydraulic system. Good luck