Without trying them out, I would simply answer 'which ever is simplest to use'.
For me the idea is to help problem solvers decide which posts are still need their attention. The no-brainer approach is usually to look if there are zero responses, but this does not always work since sometimes there are other people in the forum who politely ask for more information to help the people who may know more.
I like the approach I see on the Apple discussion forums where the poster simply sees a box at the top of the thread, with the link "Mark as answered". Clicking that link changes the associated thread icon form a question mark to a star.
To see an example of what I mean:
http://discussions.apple.com/forum.jspa?forumID=1221