Yes, better for Cornwell.  I have not re-read Cornwell within the past few months, but my recollection of what I did read is that much of what FK attributes to Cornwell is not what Cornwell wrote.  Cornwell, for instance, does take into account the basic historian's rule as stated above, of noting that anti-Jewish sentiments were common among both German Catholics and Italian Catholics, and places Pacelli's thinking in that context.  Cornwell, while very critical of Pacelli, presents a much more nuanced view than FK quotes him as presenting.