If you are doing lots of sailing short distances (like in the Bay), with lots of tacks and gybes, then you want an autopilot. If you are doing longer distances with not so may tacks and gybes, then the wind vane. If your boat is very light and will surf easily, then an autopilot. If it's heavy and you want it to track down wind in big wind, then a wind vane.

And make sure you learn the bungee cord methods of steering in this video: https://www.youtube.com/watch?v=PY1qx3PfUS0 and the associated book. I know of one guy who crossed the Pacific using this method after he read the book. This will negate the necessity of having a duplicate wind vane or autopilot.