There are some similarities between Uniteds and Citys situation and the big spending by each is actually a consequence of recent failures. Both teams are paying the price for not signing / developing the right players in the past. I think the bitterness towards City comes from the relationship between their revenue and the money spent, and the high turnover of players. Both of which are becoming less and less prominent.
City - have had the oldest squad for the past three / four years, and only since Guardiola has come in since they have signed younger players. Compare the signings of Clichy, Sagna, Kolarov, Toure, Nasri, Fernandino etc to the more recent signings of Sterling, Sane, Jesus, Stones etc. If the older signings werent so prominent in the past, then there would not be so need to spend big like this. They way City bought in recent times, they were always going to be a high turnover club, if you keep buying players who were about 27-29. But that has changed.
United - since SAF left, there has not been much long term thought, or continuity of the type of players. United have bought mediocre players or potential players in recent times. Thankfully there are still some SAF signings who were youth and potential (Jones, Smalling, De Gea) to help out a little bit, otherwise there would be even more. We have not had a decent spine of players since they all left under Moyes, and these signings will hopefully change that. A spine of De Gea, Bailly, Herrera, Pogba, Mata, Lukaka plus Mkhitaryan, Rashford, Valencia, its looking good.