Are you talking about their passives? I haven't used either (don't HAVE Dendrobium) but according to the descriptions on their units pages for this site, Dendrobium has same in-game description, but it only actually affects herself, where Cymbi's affects whole team. Pretty sure one or the other is bugged if that's the case, because 6* are supposed to have upgraded versions of 5*. If you're talking about their actual skills, Dendro's is flat-out better.