Grealish has proven he can be an absolute top class PL footballer. One of the very best in the league. I’d take that sort of “proof” over any Bundesliga achievements tbh. Didn’t Mikhtaryan have some of the best creative stats in that league at one point?
Extremely different levels of performance, Sancho had ~ 15 goals and 15 assists playing as a right winger at the age of 18/19, and then at 19/20 got 20 goals and 20 assists. On pace for over 15 goals and 15 assists again this season. He's an attacking player with excellent technique, brilliant dribbling, is a natural on the right, is a really smart and talented passer, has good decision making and has shown all these to a high level in both the Champions League and the Bundesliga since he was 18. All you gotta do is checkout his profile on fbref and see just how many passing, creativity, dribbling and progression stats he absolutely dominated and is ranked in the high 90's percentile across the main European leagues for best underlying stats.
Mkhitaryan had a consistently very good last season at Dortmund, but at United was more like the other 2 where he was up and down. He was nowhere near Sancho, was also what, 27 or so when he joined? Sancho just turned 21.
There are only 2 players 21 or under who are dead certs as they've already performed to a world class level for over 1 year, Dortmund have them both in Haaland and Sancho (Mbappe is 22). The only reason he isn't rated higher is because people don't watch the Bundesliga as much and he's a creative player, not primarily a goal scorer (and scores a very good amount anyway). Once he goes to the premier league he'll show how he's actually one of the best attacking players around and has been for the past few seasons.
Grealish is having a great season. Yes also turning 26 at the start of next season, so is 5 years older than Sancho. He's also pretty much only performed at a truly elite level this season or since lockdown, but before it wasn't close to this level. He's also a #10 or a left winger. He would also cost a lot more due to the new contract and Villa being rich and him now wanting to leave, vs Sancho's contract, wanting to come back to the prem, Dortmund not being in a great spot financially.
So between them, Sancho is 5 years younger, has regularly exceeded him in productivity playing in a top 5 league and the CL (even if you just look at this season for Grealish), Sancho is a natural on the right and equally good on the left while Grealish has pretty much never played on the right and his 2 positions are where our 2 best attacking players pretty much exclusively play, and Sancho would easily be cheaper this summer. It's a no brainer between them and if Sancho played in the prem, people would view it as a no brainer. Not to say Grealish isn't quality, just he doesn't make sense for us and certainly not ahead of Sancho.