I'd rather take my chances with D'Arnaud.
Not too many great catchers out there and this kid is the best catching prospect in baseball. A great catcher does a lot more for a team than a stud outfielder. He's the backbone.
Stanton is a beast though. He's stil cheap. Marlins won't move him unless they get a LOT of prospects/value in return. Guy who can hit it 500 feet puts fannies in the seats.
Marlins will not trade their top player within the division
If they do move him it will be to most likely an AL team. Now once that trade goes down could the Mets try to get him from the new team after some time.
The best situation will be after a few years Mets (may) have some cash and with the young core coming add him as a FA