That's pretty much the difference - United have invested heavily on world class players definitely bound to improve the squad and ones who're guaranteed to start in the XI, while Mancini's just invested heavily for spending's sake.
I mean - Scott Sinclair (7M), Jack Rodwell (15M), Javi Garcia (20M), Maicon (3.5M) have all barely even started for City this season! On hindsight, Matija Nastasic for 15M seems the only signing close to being solid, and even he's started only 12 league games this season!
Sounds similar to Brendan Rodgers' case in Liverpool, where Sahin, Assaidi have all been signed only to warm the benches. Joe Allen for 15M (needless to say) is overpriced, and Borini for 10M sounds no better.
Disagree that Mancini spent for spending sake, because we lost 3 bench players in that window and replaced them with 3 other bench players...
Johnson out, Sinclair in. (downgrade)
de Jong out, Rodwell in. (downgrade)
Savic out, Nastasic in. (upgrade)
Now let's look at the actual "improvements" to the squad. Javi Garcia, 3rd string target behind Javi overpriced Martinez and snub de Rossi. Maicon, worked with Mancini at Inter and for 3.5m he's a player worth having imo, he's different from all our other fullbacks, he's basically a wing back and I strongly believe Mancini brought him in to wreck havoc in that 3-5-2 system of his.
While I agree our signings haven't been good, they haven't been bad either and they got a real excuse they could play on you, injury. Maicon(still injured), Rodwell(still injured) and Garcia have all had their fair share of injuries(Maicon and Rodwell more so), it just hasn't been an easy switch for any of them.