Hummels and Subotic are both excellent defenders however Hummels is the more technical of the 2 and can also play in midfield. If City are interested in either of these then there are a number of options bearing in mind the wealth the club has.
Option 1: Go for both of them for say £50-60 Million and chuck in Kolo if needed. Then play both together from the off next season and move Kompany into midfield and put Barry on the bench. This would see Kolo probably 2nd or even 3rd choice at either RB or CB!!
Option 2: As above but use Boateng if needed to complete the deal. Kolo becomes cover at right back.
Option 3: Buy Subotic only. This depends on how you view Kompany's playing style and whether you see him as Hummels. CB pairing Kompany and Subotic, Boateng and Subotic. If we buy him he is a definite starter at the behest of Kolo.
Option 4: Buy Hummels only. This depends on how you view Kompany's playing style and whether you see him as Subotic. CB pair could be Boateng and Hummels, Hummels and Kompany. If we buy him he is a definite starter at the behest of Kolo.
Hummels and Subotic are so good together but I would say that Subotic cleans up any mistakes made by Hummels when defending. Hummels is more versatile and could play in midfield so maybe he edges it especially as I see Barry as the weakest link in midfield and he's not getting any younger.
Assuming the following leave in the summer: Given, Taylor, Bridge, Lescott, Onouha, Vierra, SWP, Ade, Jo, Caicedo, Santa Cruz, Belleamy and who knows if any of these will survive the summer clear out: Logan, Vidal, M Johnson, Boyata, Cunningham and Nimely. We will have enough from the sales to go for both!
Personally I would like to see a defensive squad of Richards, Boateng, Hummels, Subotic, Coentrao, Kolarov, Zab, Boyata, Kolo but to get the missing palyers will take some doing a lot of cash and at least a top 3 finish this season