I like Grubauer too but I think the time to trade for him came and went with the draft. Not so sure the Capitals want picks in next year's draft for him, nor would they be in a hurry to trade him to an eastern conference team. That being said, I would also prefer him to Bernier.
If you are strictly looking at the FA market though, there is nobody who is clearly head and shoulders above Bernier and that includes Mason. I would rank Mason as the best FA goalie and Bernier second but the gap isn't all that big in my opinion. Just my opinion but Mason is a slightly above average starting goalie in this league who some fans have convinced themselves into believing is much better than that.