Consider strength but don't ignore durability. From childhood experience--man, that's been a while--whatever you use needs to contain a bearing surface that's intended to be used in this or some similar application. Don't just put the end link of a chain in a u-bolt no matter how big they are. Sooner or later it will fail and by default, there will be someone in the swing when it does. Get a connector with a bearing in it that's intended to allow motion with minimal friction and wear.
DB
deja moo: I've heard that BS before.