I've made this point before when it's come up but I think Tierney is the one who'd be more versatile of the two and probably closer in comparison to Alaba & Kimmich than Robertson.
Robbo's style of play better suits him being in wide areas and if he's said he's uncomfortable at wing back, then I can't imagine how he'd feel in CM.
Tierney on the other hand has comfortably played LB, RB, LM and the left side of a back 3 so could see him fit into the left side of a midfield 3 with the same ease