Re:
They're pretty similar, jockey wheels being a difference. The m900 uses a sealed bearing lower (tensions) and it's really nice, look after them
You can't dismiss jockies they are a main rotating part, it's an easy upgrade for other mechs though, not a cheap one. Same top/guide pully
It has the potential to drop more weight as the cage is bolted on differently and this can be switch to Ti or Alu and is a big chunky bolt like the mounting bolt.
Other little aesthetic differences but that is preference.
Of course m900 is medium, where XT is short or long cage.
As a side the weight chart above doesn't quite seem to show like for like sizes for everything.