Fred is comfortably the better midfielder. Faster and stronger are significantly down the list in terms of important attributes for being a midfielder. Good to have of course, but there's a whole lot of things that are more important. Fred's better at most of them.
In saying that it probably does make more sense squad-wise for Fred to be the one sold if it's only one of them. ETH sees Scott as his only backup for Casemiro whereas we have more options in Fred's position (plus his age and contract situation). Hopefully we do bring in somebody else though (or ETH thinks Mainoo is truly ready to be that backup option) so we can sell Scott as well. Preferably someone who can play both instead or next to Casemiro depending on what we need in different games.
For me it doesn't matter Fred is erratic and often is out of position.
One big thing to note for me is that I think the whole scouting department and also people that analyze the data at United will look at the profiles of players and decide Fred can't do X, Y, Z so what players can we bring in that will do that job.
We need engines that can press to create chances higher up the pitch, Onana was a missing piece, Hojlund presses from the front, so does Antony, Rashford, Bruno, Casemiro, Mount. We want that kind of players, Amrabat is also a workhorse that has end product too and when you look the bench he will be an able player that can add some more steel to the midfield.
I think teams will hate playing United next year if we can get all our targets. Moving on Fred, Maguire, Martial is critical at this point, if good money comes in for McT he can leave too but I've seen rumours of VDB to Sociedad too.