IMO if we sign Marcos then the prospects for Benatia are dim. From the look of it, the reason Marcos is considering moving is because he's now 3rd choice CB after Luiz; if Benatia becomes a Barca player he would end up in the same situation that he was trying to get away from.
Besides, if we can land Marcos I don't think we should be chasing Benatia any longer. This is probably not a very popular opinion on this board, but think about it from a longterm perspective. With both Benatia and Marcos in the squad we will have tremendous quality and depth, but it will also mean that Benatia will be the starting CB for at least the next 2-3 years which will derail Marcos development. Either that, or Marcos establishes himself as a starter and we end up having spent 40m or so on a bench player.
Marcos is definitely not on the same level as Benatia, for now. Our defense has been frustrating for so long, and with Marcos it may continue to be so for a while until he develops. But we're not going to rebuild this team overnight and I would rather we select the right materials rather than go for what is expedient.