I agree that there is no standard for X11 pasting. I expect the middle button to paste the current selection and if that's not what happens, I'll usually change the program I'm using rather than change my behavior. Vim, Emacs, KDevelop, and Firefox (forms) all give me ddefef.

That said, I think behavior 3 is interesting and potentially useful but I would want to see that as an option rather than the default. I use selections a lot and it would bug me to paste my last firefox search term rather than the current selection.

Edit: I meant 3 not 1.