Thanks, great responses. What about expense and maintenance? I assume they'd be more for a second mast?
Edit: Also, does size of the boat come into consideration?
Basically the costs are doubled in theory, but isn't usually reflected in the purchase price (of used); a sloop is a sloop and a ketch is a ketch. We recently had our sails cleans and repaired and the mizzen was half the price of the main and a quarter the price of the genoa. Again, it's about square footage when it comes to the canvas and the rigging as the mizzen's rigging is half of the main's. Size, in terms of length is less important than displacement where hull ratios define rigging type. Generally what one sees is that the heavier displacement boats have more sticks, which translates to more sail and power closer to the center of gravity; keeping in mind most hulls are fastest when closer to vertical. I've been on boats where cutter, sloop and ketch rigging were available on the same hull; the ketch sailed the best, especially when cruising. An example is the Gulfstar 50 CC, while being 50' is relatively light and sails just fine as a sloop, but came rigged for cruisering as a ketch.