I absolutely adore both aircraft, but in terms of pure performance, the late model P-38s for sure. The P-38L outpreforms the Mosquito Mk. FB VI in every way, except possibly turn circle, but I'm not sure about that. Not to mention things like boosted ailerons and dive recovery flaps that were included on late model P-38s.
However, that is not to say the Mosquito was not excellent at some things the Lightning simply could not do. Including, but not limited to: anti submarine operations with a 57 mm cannon, proper bombing, and much more.
In terms of performance, the P-38. In terms of utility, probably the Mosquito.
In the end, they are some of the most fantastic preforming, and beautiful aircraft of the war.