Assuming you mean the Four Seasons in Brickell, I'd easily pick the JW on location. While the hotel may not have been renovated in the last year, it's still in good shape, and the gym is excellent. The MR Biscayne Bay is going to feel much further away, and it is in a much less interesting part of town. (You could walk to the JW, but not so much to the MR.) You're going to get tired of spending that much time on Metromover or in an Uber.

The non-Marquis JW is even closer, if that's an option.