I agree with previos poster's points but would add that you look to see if the oil can be changed in the saildrive without hauling out. Older Volvo models (like my MS25S) can only be drained from the bottom of the leg. Newer models and some other brands can be pumped out from the top of the Gbox.
Another advantage is that they are usually installed so the prop is further forward than a shaft and therefore are less prone to catch ropes over the side or dinghy painters when reversing.
Ray
saildrives eliminate all shaft alignment problems and there is no stern gland to worry about, no cutless bearings either. when fitting, you simply glass the supplied plinth into the hull, cut the hole for the leg and drop the unit in.
when you say 'sterndrive i presume you mean a shaft drive, not a powerboat sterndrive??
a point to remember is that if an engine requires a right-handed prop, with a saildrive it will need a left handed one.