It's hard to make a comparison like this, with any two types of WWII fighters. To compare the P-51 and P-38 is hard unless you largely narrow down the categories. The P-51 was a better all round fighter (air to air), but the P-38 was definitely a force to be reckoned with. The P-38 had very good turning and climb performance at altitudes above 20,000 feet, but could not dive as well as anything due to problems with compressibility in most variants. The P-51 was a "better" performer than the P-38 at all altitudes, but above 20,000 ft as mentioned this gap shrunk considerably. The achievements of the mustang in the ETO were nothing short of amazing but again you have to look at all the factors involved. Rather than write a book here, I will say that both fighters along with the P-47 were outstanding, and any one of them was capable of winning the war. I look at combat experience, and achievements when comparing fighter aircraft types (at least when discussing war topics) and its hard to choose one over the other in such a large envelope of categories.