I saw Mountcastle a little in ST 1.0. His arm looks much worse than DJ. DJ's arm in left is probably average. Mountcastle's arm is not close to average anywhere on the field but DH. I will say my looks have been limited but what I have read and seen bears this out so far.
DJ has more experience and a better arm but overall the bats will probably decide who get the playing time.
Where does this come from?
Mountcastle came up as a SS and although his arm is not ML caliber at SS, DJ Stewart has one of the worst arms I have ever seen. I haven't watched Mountcastle in person but I don't think it is possible for him to have a worse arm than DJ
If that's the case, I'm not sure I bother carrying three catchers. Maybe for the 30-man, but definitely dropping one the first time the roster contracts. I get the demand of the position, but I'm not sure having that much protection is needed given it's one position out of 10. I get you could potentially use a catcher in the DH spot, but not sure any of our catchers have shown we need to keep their bats in the lineup when they're not behind the dish.