The problem with the Silva v Hummels debate is while Silva is a better fit, Hummels is a much easier buy.
Silva cost PSG 42 mil last summer, so they will easily want 50+ to make this deal even possible. Hummels on the other hand could reasonably go for half of that at Dortmund, I'd say 32 is fair.
I'd much rather see us get for example Hummels and say Inigo for around 50 than just Silva.
Piqué-Hummels could work bc both like to press high up the pitch, which makes running the trap much easier. The thing that impresses me the most with Hummels is his distribution. His outlet passes from defense to the midfield and long balls up top are fantastic, IMO the best in the world for a CB.
With Silva he is a better tackler, interceptor and tactically aware, much more of a leader that Pique could really benefit from too.
IMO tho, Martinez should be purchased regardless of who we get. Kid has that Puyol-esq tenacity to him, something that neither Hummels, Silva nor Piqué have. He is young and would be very cheap, plus he has La Liga experience.
With Hummels, Inigo and Neymar plus the unleashing of the Deu, could be a very fun season next year.