I prefer either a Cow's Hitch in a single line, or two bowlines on separate jib sheets. On certain sail clews I have seen a cows hitch slip so you need to try it for your specific clew but 90+% of the time I have never seen a cows hitch slip. It is also smaller in mass and leaves less of a welt than two bowlines.
Cow's Hitch
I don't understand Don's comment on a bowline being tough to get untied? I have been on a commercial fishing boat where we towed another boat over ten miles and under tremendous loads and the bowline came undone with zero issues or effort. Simply "break the back" and it comes untied..?
What ever you do avoid the use of shackles. The clew is heavy enough to do some damage you don't need to add to it. Those of us who have had the opportunity to be hit by a flailing clew understand this well.
Many racers won't even tie stopper knots to avoid getting whipped. It also speeds up sheet changes from inboard tracks to outboard. If racing, the lightest mass at the clew possible is what is generally, preferred especially in light winds, and this is where a cows hitch will win over the bowline in light air performance. Lighter weight performance sheets will do more but starting with a low mass knot is a good first step..