Sitting deep to nullify Robertson is by far the lesser evil of the two. If you try to pressurise Trent's space, it opens up the rest of the pitch to Robertson and our quick forwards. If you sit deep and allow Trent to pick his spot, the spot he picks will still be attacked by our small forwards. Trent is a threat either way, but the first plays to our other strengths, while the second does not. Unless of course it's a set piece that allows our CBs to attack the ball.
I think you have highlighted one of the secrets of our system though. Sitting deep and stopping Robbo's rampaging runs seems fine at first but the Achilles heel is that you allow Mane to push up one on one on your full back. Which means Mane will just stand still, suck you in and then burn you for acceleration.
Same on the other side defend deep on Mo to stop him running in behind and you offer Trent space. Push up on Trent and you give Mo space. How about shuffling across so you can press both of them. Job done ?
Unfortunately not, Trent just hits a long diagonal and suddenly it is Robbo and Mane ripping you apart.