In the words of Art Heiden who flew both the P-51 and the P-38 in combat " There's nothing a P-51 can do that a P-38L can't do better".
The P-38L flew further, climed 30% faster, flew higher, carried more, flew marginly faster and turned better. The P-38L also was more complicated to fly and cost more, for an average pilot the P-51 was easier to fly.