I feel it's impossible to choose between the two. It all comes down to the alternatives for the player we don't buy.
de Ligt's case is easier as he comes as a direct Piqué replacement. Are there any other obvious CB talents?
Frenkie's got more competition in the CM-roles, but at the same time he could be the Busquets replacement.
I still believe Coutinho - Arthur is our destiny at CM. Watching City successfully play with David and Bernardo Silva renewed that belief.