The MBT has only one downside really, and it's more personal preference based...
The trigger is cut from flat stock, and the trigger face has a more squared off profile at the edge. Some find this annoying.
It doesn't bother me personally, but I do prefer the feel of the Geissele trigger face.
Other than that, you have to spend a lot more for a Geissele that is significantly better than the MBT.
Either are great triggers.
Really it comes down to desires pull weight on what direction you go.
I prefer two stage triggers, as I feel they are safer when getting into lower trigger pull weights.