What are the performance and behavior differences between a full keel and a fin keel on a sailboat, and when would I want to choose one over the other?
A full keel is very stable and gives you better tracking through seas - you don't need to adjust your helm much as you crest waves.
A fin keel loses some of this stability, but you gain manoeuvrability - the shorter keel lets you turn much more sharply.
Keels on boats are designed to offset the push of the wind on the sails by transferring the lateral force into a forward motion. The boat can't go sideways because of the keel, so it goes forward. In addition, the wind will push the boat hull over- rotating it to leeward, and the keel will balance that force to keep the boat upright. Full keel boats "track" better- they sideslip less- you'll be able to steer in a straighter line and your autopilot will work less, but give up speed and ability to "point" high due to drag. Fin keel boats were designed to reduce drag, and while they are faster & point higher, the keels are deeper in the water, and less able to operate (and anchor) in shallow water. Full keel boats tend to be slower, behave better in bad weather, are heavier. Fin keel boats tend to be faster, lighter, and more "active" in weather.