the difference between Grandal and Realmuto's framing according to Baseball Prospectus amounts to approximately 16 runs which is equivalent to 1.6 wins.
The difference in their fWar is 1.2 wins (4.8 for Realmuto career high in his age 27 season, 3.6 for Grandal career high in his age 29 season). Realmuto was good for 3.8 WAR and 3.7 WAR in the preceding seasons, meaning his baseline is better than Grandal's career peak at this point.
If you want to studiously accept the math that Baseball Prospectus advocates, that means Grandal was a very slightly better player in 2018. Now that overlooks the other aspects of Prospectus's calculation of defense but I'm not sure to what degree Prospectus's calculations might replicate fangraphs defensive WAR calculation so I will ignore that. If you did not ignore it they would be roughly the same value.
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Grandal is two years older and much more likely to hit his decline immediately. Regression to prior career norms would be a reasonable expectation at 30 if nothing else.
Signing him likely requires a commitment for 4-5 years at top of the market catcher salaries (he should be highly coveted on this market by one of the numerous big market teams that could use catching help) and most probably forfeiting a draft pick. You are certainly going to expect decline over the course of that contract and you will be owing him much more money than he is worth, most likely, at the very time your young core pieces are hitting the expensive years of arbitration and other pieces might be hitting free agency.
Realmuto is going to be comparatively cheap in terms of salary, he will require no post-prime commitment. he will require an investment of prospect equity, though whether that would actually end up hurting is unclear. It remains to be seen where the Marlins are in terms of reasonableness at this point.
Maldonado is credited with perhaps being worth .6 framing WAR, to go along with his rough 1 WAR production over the last few years, leaving him a considerably lesser value as a player than Realmuto or Grandal, though one would also suspect considerably cheaper.