I think both are a bit overrated - definitely good players and at least rotational players on a top side, but its not clear to me that either is a player that you really want in the starting XI if you're a team looking to win the Premier League and/or go deep in the CL.
A lot of the issue is their profile - heavily right footed players that want the ball at their feet and are either going to play on the left wing or behind the striker. Its hard to get into top clubs with that profile because there are only really two spots at most (sometimes only one) that you fit in any tactical setup and the competition among players with that profile at top clubs is pretty fierce.
I feel like Grealish has a more well rounded skill set and so you could see him adding some more versatility to his game as he matures (playing a bit deeper or being able to do more things in the buildup). So I'd probably pick him in a vacuum.
For United, I don't think you can comfortably fit Rashford, Bruno, and either of them into the same starting XI without resorting to some kind of fantasy football alignment. Competition for places is a good thing so that wouldn't dissuade me from buying one of them. But this isn't really the profile that United needs in the starting XI assuming that Bruno comes. You really need a left footed playmaker who can cut in from the right side, a la Mahrez or Bernardo Silva, or a right wide forward that is a direct goal threat, much more than either Grealish or Maddison. Of course, players like that are harder to find.