Van de Ven is undoubtedly the more valuable and appreciating asset—he's younger, has four years remaining on his contract, and is on lower wages. A straight swap would be excellent business for City, but not for Tottenham. I highly doubt Levy would be so naive as to agree to a straight swap for Grealish, who not only commands higher wages but will also be 30 by the time he arrives at Spurs.
If City manages to pull off a straight swap, I would be absolutely wowed! It would be nothing short of a masterstroke in negotiations. Considering the circumstances, it would feel like a huge win for City, especially given the difference in age, contract length, and future value of the players involved.