While Nedkov is a wrestler by trade, but both have takedown chops.
Stanislav Nedkov is more of jiu-jitsu practitioner these days, but has used some takedowns in the past. Not much has been seen with his takedown defense, but it will surely be tested against Thiago Silva.
Silva does not have a decorated wrestling background, but he no doubt has underrated takedowns. He is powerful, strong and changes levels quickly to put opponents on their backs.
Although he failed a drug test for the fight, use his clash with Brandon Vera for example. He used power takedowns for three rounds while dropping heavy ground and pound.
Nedkov's background would seemingly give him the edge, but with MMA, takedowns have to be set up with striking and timing. That is where Silva will likely be better, just slightly.