I dont think Esson was put back into the team cos Reguero was performing poorly. Esson returning to the team correlates with the time Foran was out injured. I think Esson was reinstated as a source of leadership and experience, he also captained the team during that time. Butcher's never chopped and changed goalkeepers that much so when Foran returned i think he just kept it going for the sake of stability, just unfortunate that Esson got injured again.
I think both of them are pretty equal qualitywise, Reguero probably edges it cos we had our best form with him between the sticks but its probably the most tricky choice in terms of team selection for me.