When it comes to the subs bench, City will have the advantage because as you rightly point out, they have strength in depth and thus can put out a very strong sub bench which they have done in the past and as we see week in week out, subs coming on can make a huge difference to a game. Ten Haag will not want the game to go to extra time because he knows he does not have the strength in subs that City will have. Fresh legs will make all the difference and City have been able to put some extremely experience players on their sub bench which I am sure will worry United's manager.Unfortunately, City have strength in depth; injuries don't matter quite as much to them, - or, rather, do not have as much of an effect as would be the case with such injuries with other teams - because they have a sufficient number of quality players - players who would stroll into a first team with any other club - on their bench.
City's 6-3 league hammering of United should not be looked into too deeply when looking at the two teams because Ten Haag only came into the club in April 2022 and City beat them in October 2022 whilst Haag was finding his feet with the club. The game in January could be seen as being the game with interest because Haag had by then a good few months under his belt with the team and United beat City 2-1. So, even with an understrength side to that of City and no where near the strength in depth of City, United beat them in January. In my opinion City will be going in as clear favorites but this will be the first time both teams have meet since the United win in January so even with all the strength and skill that City have, I cannot see it being an easy game for City, unless United do what they did at Liverpool which is down tools and got hammered 7-0 lol. You never know