Adrian Newey or James Allison: Who is a better technical director in F1?

With the contract extensions of James Allison and Toto Wolff, Mercedes have fixed their top staff members. This ensures continuity and gives the German team the best possible people on board. Team boss Wolff even calls his technical director Allison "the very best". Is that true, or is Red Bull Racing's Adrian Newey a better technical director?