Honestly I wouldn't want either of them. Don't take me wrong both are brilliant defenders but they would cost a bomb which means they will be expected to hit the ground running. At nearly 28 its only a matter of time that Koulibaly will start showing some wear and tear which isn't ideal either.
In my opinion we should take a holistic approach rather then expect a miracle worker
A- We should ensure that DDG stays. If we fail in that then let us involve him in some swap deal with Courtious. We need a top goalkeeper whose EPL proven
B- Lets get some top Fullbacks. This failed wingers as FB thing is a bit silly tbh. I'd go for Tierney and Milenkovic. The former is an old fashioned EPL type of FB who'll fight for the ball for the entire 90 minutes while the latter is comfortable with the ball, he can attack and defend, he provide inches (something only Smalling provides) + he can easily move as CB which would be great if Dalot ever becomes WC.
C- Smalling is literally the only CB we've got who provide inches. So I'd say lets get Andersen and have that issue sorted once and for all.
With those sorted we can get rid of Rojo, Jones, Valencia, Bailly and Darmian.
So the defence would be
RB: Milenkovic, Dalot, Young
LB: Tierney, Shaw
CB: Smalling, J Andersen, Lindelof, Tuanzebe
We would have a perfectly balanced defence there with all the necessary inches, physicality and ball playing BS everyone keep rambling about. There's also 2 future captain material there (Tuanzebe and Tierney) as well.