Seaworthy, the ability to cope with the given conditions, more seaworthy, able to cope with a broader range of conditions. For extreme conditions I'll take the ski, no contest, as for speed, the entry level ski's I was suggesting are about 40 to 50% quicker at cruising speed than most sea kayaks let alone SOT's, they have a much more efficient hull design and will take less energy to paddle. The longer skis, 19ft, 20ft and 21ft are not relevant here, they are the domain of experienced paddlers only, either that or people with really really, good balance. I take it, Boatman, that with Cornwall as your location you have more experience than most with surfskis, they're not as common outside the south, although that is changing, with racing up here getting more popular and the ski being the tool of choice. I don't know how up to date you are on the current ski's but the new plastic boats are fantastic, ideal for beginners or as a rough weather playboat, river runner or surf toy for the more experienced. I would never dream of recommending a closed cockpit boat to anyone now, the ski is much safer, with no requirement to master the roll, self rescue is a far easier process. There may be a case for the sea kayak being more seaworthy in very extreme conditions, but that is outside the ability range of most mere mortals and certainly long past the point the SOT paddler became a swimmer.
For the purposes of the original post an entry level ski makes a good alternative to a SOT, a sea kayak does the job as well but needs more kit and training to be used safely.
