I used my neighbor's Wagner sprayer to do my fence. It worked great and was fast. I did mine in about 6 hours. Downsides were that I had to re-fill the container after about 6 boards (one side only of course on a 4ft privacy fence), I was completely coated with stain when I was done, and even though it was a calm day, I still had to protect things with tarps pretty well to prevent drift. My wife did the parts close to the house with a brush while I sprayed.
I used a product that is called Ready Seal and it worked great! It's motto is that it is "Goof Proof" so runs or heavy spots were no problem at all. It is a stain and sealer in one, so that is nice too.