The SpaceX vehicle may look like the safest choice. But is it? A handful of manned flights does not prove that a vehicle is 'safe'. Likewise, some anomalies in 1 flight do not prove 'unsafe'.
Perhaps the knowledge of Starliner issues makes it more accurate to assess the risk.
Either vehicle could lose the crew on the return. "Astronauting" is a risky business.
I do not envy anyone on the decision chain.