With new data showing similar outcomes of mismatched unrelated donor and haploidentical related donor allogeneic transplants using post transplant cyclophosphamide, how does one decide which donor is optimal?

It is certainly an unresolved question, and the short answer is that either would be an acceptable option with post-transplant cyclophosphamide (and I have used both). All recipients of mismatched donor transplants should be tested for the presence of donor specific antibodies, and their presence could be one factor to impact the choice of a given mismatched MUD vs a haploidentical donor. Another potential factor could be the relative urgency of proceeding to transplant, as related donors can be often identified and collected with somewhat greater expediency than unrelated donors. Factors such as donor age, CMV status, and ABO compatibility could also impact donor selection.