We've clarified the menu button labels for your convenience.
Forum Posts Member View is for ... Members! It returns ALL new posts since your last login, including the Members Only forums. Of course you must be a registered member to use this search. (It's the old "New Posts" search)
Forum Posts Guest View is for guests. It Does Not Show Members Only forum posts. (It's the old "active discussions" button)
ETA: if you've bookmarked either search your link should still work fine.
ETA: Still fiddling around with the labels...