A comparison of Silva, Hummels, J. Martinez, and Vertonghen.
Experience - Silva is the most experienced defender, with a few more years under his belt, which shows in how he plays. He is often able to stomp out attacks before they really get going. The other 3 have similar experience and are of a similar age. Martinez has featured regularly in the first team since he arrived in Bilbao in 2006 and perhaps understands how barca play to greater degree, but he played most of his career as a DM, not a CB. It might also be wise to mention Vertonghen serves as captain of Ajax which can speak to his maturity as a player and leader.
Positions - Both Hummels and Silva really only serve and CB's. Silva again due to his experience might be the best CB of the 4 but all of these players have the talent and ability to make a big difference in any game. Martinez and Vertonghen both are able to play as defensive midfielders, which can be a valuable asset if Keita leaves Barca, and if other players get injured. Vertonghen can also play as LB, a position Barca need to fill and although Alba (now also Alaba) seems to be the favorite, a player who can fill in for other players when they get injured or from fatigue is a great thing to have. Vertonghen can also attack when the situation call for it, but so can Silva and Martinez (not sure about Hummels).
Age - Silva is the oldest of the lot. Currently 27, and will turn 28 in September. May seem to think he is too old, but defenders can often last a while and we can assume he can last at least until he is 34 then we have 6 years with him, if not longer which is not too crazy to assume. The rest are all in the same age range with Vertonghen just having turned 25, J. Martinez being 23, turning 24 in September, and Hummels also 23 with a birthday in December. Age, at least for me is no issue with any of these players.
Ability/Physique - All of the players can go on the offensive at times which can be useful.
Height being the issue to seems to be for Barca, it can be important to get a few extra centimeters on the field. Silva - 1.83, Vertonghen - 1.89, Martinez - 1.90, Hummels - 1.91. All of these players are also strong physically.
Their ability to play is the most important issue if Barca want to buy them, and will probably be more of a factor than price. Vertonghen istechnically adept, intelligent, adaptable and good in the air, with a powerful shot, but being fairly attacking minded he could leave gaps in the back at times. Javi Martinez seems similar in style as Vertonghen but perhaps better going forward. He can also sometimes get caught out in regards to his positioning. Silva is the best defender of the 4 in my mind, and can also get forward when it is called and is perhaps more aware that when he gets forward there may be gaps left in the defense and is more tuned in to that issue. He also has great positioning and forsight. Hummels can help in offense but is from what I've seen the least attacking minded of these players, but is superb i defense, which is shown by Dortmunds stats this season in regards to how little other teams are able to score against them.
Price - Silva is probably the most expensive of the four and will take a sizable chunk out of the transfer budget no matter how large is might be, but his price might also be alleviated by adding Keita to the transfer, but will still be a large sum(35m+?). Hummels seems to have a price of around 30m. around 30m for Javi Martinez seems to be the going rate aswell. Vertonghen is the cheapest choice as he is the only one who seems to want to leave his current club, and could go for a fraction of the asking price of the other 3. (10-15m?)
Silva is in my eyes the best choice if money was no issue, but it is. He is a great player, but will come at a great expense. Hummels and J.Martinez are perhaps cheaper and younger than Silva, but not much cheaper and with less experience. Hummels and Thiago Silva also do not seem to have any intention of leaving their current club. J.Martinez I dont have to much info on if he wants to stay or leave, but is perhaps interested if the right offer comes along. Vertonghen is fairly cheap, he has at least as much experience and Hummels and Martinez and most importantly he want to have a new challenge, but to get him Barca might have to move on him soon as teams like Tottenham and Arsenal are interested in him, and the latter rumored to be ready to spend big to get back to their former winning ways.