Statistically Girard is the best transition dman in the league.
Girard is better at skating the puck out of the zone. One of the best in the league. Byram is better at outlet passes on the breakout. Also one of the best in the league.
That said I think Cale is clearly better at skating the puck out of the zone than G. He just usually gets matched up against opposing top lines because he's usually playing with the Avs top line. Which would make it harder to skate out of the zone and skew the stats in Girard's favor.
G is a great puck moving defenseman but a lot of of his (the whole D really) analytics are misleading. Especially the defensive ones, which are impacted by the Avs playing in the O zone more than any other team, and limiting scoring chances that way, as opposed to actual ability to defend.