I prefer Visor for a few reasons. First, I just prefer Terminal to iTerm (especially now that it has tabs). Second, F3 (my hotkey) always gets me Visor; cmd-tab may take more or less key presses depending on what apps I've been using. Third, since I set Visor up to use a different Terminal which runs in the background, so that's one less app to cmd-tab past. Mostly, it's a convenience thing-one F3, and always one F3.
