The Difference being, of course, that in this case, the character will have the chalice of Nectar which makes casting significantly more reliable. Throne of vines is of little use on a Branchwraith in this case, unless no more woods are needed, in which case it can be used to make the summoning even less likely to fail. It also increases the range of the summoning by 6", right? For 40 points that could be nice.
Then there could be a plan B use for it, making a Wychbomb in case of an MSU army.