To me, it just really depends on how my head and stomach feel.  Sometimes I really want sweet tea, and sometimes I don't want anything with sugar in it because it'll make me sick.  Unsweetened tea is just a little bitter, but sometimes it's just what I want.  Unfortunately, unsweet tea will mold a lot faster than sweet tea in those big metal urns (Don't ask me why this is) so I usually have to check it.  After about four hours, it's off and needs to be remade.