There actually isn’t much difference. I’ve said this many times but people on here are very fixated on starting positions. A game is very fluid. If anyone believes 10 outfield players will remain in their starting positions throughout 90 mins really needs to observe a match far more closely. Not only is it unrealistic, it would allow opposition to just make a few tactical alterations and completely control a game.
Let’s take 4-3-2-1.
When no one has control of a match and the bulk of play is in the center circle do we think that the AM will continue to stand in the uncontested area of the pitch crying at his team mates for the ball (like Bruno) or will he naturally drop deeper and make it a flat 3 midfield or even a deep 2-1 or 1-2?
You don’t control a game. Your gonna scream at your wide forwards to drop deeper. Suddenly your playing 4-5-1 or 4-1-4-1. Possibly even making a 5-4-1 if one side is being attacked heavily and a winger has to become a fb for 5-10 mins.
In United’s case, I imagine (I don’t watch you unless your on tv at my job so it’s all theory) having 2 midfielders slightly deeper allows Bruno to become a forward when you have momentum. This it becomes almost 4-2-4.
Liverpool play “4-3-3” but really they don’t. Since their fbs average pitch positions are midfield. Fabinho drops into defense and Salah goes inside and it becomes more of a 3-5-2 or he stays wide and it’s 3-4-3.
I could go on and on but systems as you of them are archaic and it’s not how modern football is played or coached.
Again, I don't meant to sound like a dick or anything but neither you nor the person I initially quoted are saying anything here that is revelatory or means the discussion in this thread isn't one worth having (the first guy's post was particularly abrasive in its wording).
Nobody's saying all ten outfielders line up in two specific shapes for the entire 90 minutes if we change from 4-2-3-1 to 4-3-3 and boom, one leads to better football than the other. If you want to talk about there not being much difference, the OP specifically highlights how we changed between two systems under both Van Gaal and Mourinho, and it led to visible changes in how we approached games, as well as our performances (and with all due respect, there's a lot more to it than the bit I've bolded in your post and really just comes down to having watched us play every week for the last 8 years - which you've already said you haven't):
1) Van Gaal, after mixing and matching formations and systems in his first season and experimenting with three at the back for most of it, settled on a 4-3-3 shape with one midfielder (Carrick) sitting deep with two in front of him playing very different roles - Fellaini (on the left of the "triangle") was pretty much a target man in midfield, while Herrera was more of a classic shuttler on the right. It led to some very good football (the mythical 4-game spell where we won convincingly against Liverpool and Man City) and one of the very few spells since Ferguson's retirement where we looked balanced in attack and used both sides of the pitch well, with both fullbacks and both advanced midfielders contributing heavily... and then he switched the system the next season to an extremely tumescent 4-2-3-1 with the two sitting midfielders and both fullbacks playing extremely conservatively. Cue a season of utter garbage football where we scored 49 league goals.
2) Mourinho, again, started his first season with us playing 4-2-3-1 / 4-4-2 with two midfielders holding their position behind Rooney as a number ten or second striker. This led almost immediately to questions over whether this was the best shape to use to get the most out of our best players at the time: Zlatan, Martial, and most importantly Pogba, who had played his best football for Juve in a more fluid role with less defensive responsibility than we were giving him by putting him next to Fellaini in a double pivot. Sure enough, we did eventually switch the shape up and played most of our best football (going on the famous "unbeaten run") in a similar shape to Van Gaal's 2014/15 one - Carrick sitting deep and often dropping into defence, with Pogba occupying the left half-space and Herrera on the right (from where he contributed an almost uncanny number of decisive moments in games). Then again, just like Van Gaal before him, Mourinho ditched this system again at the start of 2017/18 and went back to two deeper midfielders and a number ten (Mkhitaryan, replaced by Lingard later) behind the striker.
The discussion here is that a lot of fans have noticed we've played better football and seemingly gotten better results with one system over the other across a period of at least three seasons,
and one of them was very clearly more conducive to getting the best out of our star midfielder in this time. The question is why Mourinho, Van Gaal, and Solskjaer (in the first half of 2019/20, before we signed Fernandes) kept reverting to the system seemingly less suited to us - my personal take is that they all just didn't trust the defenders (and later the goalkeeper) and wanted the security of an extra sitting midfielder in front of the defence to help protect us in transitions.
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