Autopilots and windvanes are good at different things so it really depends on what you what it to do. Autopilots are good at steering the boat in a straight line and can sometimes do fancier stuff like tacking or following waypoints. The downside is they are expensive, not totally reliable and draw a lot of power. Wind vane units are best at reaching points of sail without too much swell and enough wind.
If you are using the unit to steer while you read a book in the cockpit and pay somewhat attention, then a windvane will work really well. However, if you are getting the unit to do things like tack for you so that you can crank in the jib sheet, an autopilot is the way to go. On boats with autopilots, I actually use the wheel lock a lot more than the autopilot. That is because the wheel lock is good enough for the quick trip to the foredeck that I need to make and once we are sailing, I much prefer to hand steer. If you are looking for a unit that can steer while you go forward to raise sails, an autopilot will be better.
One other thing to note is how both of them install. For wheels, the autopilot either goes down below on the quadrant or on the wheel itself and for tillers, it is essentially a tiller extension. Wind vanes take much more structural work back aft but they do have the advantage of having no wiring.