The P-38 definately had advantages but as always, my money is on the pilot not the plane. Take an "average" pilot on a high altitude bomber escort mission and I'll bet on the P-51.
I've read a ton of unflatering things about the p-38. Here is an example from Wiki:
>> Although the P-38 had gained an enviable reputation in North Africa as the "fork-tailed devil," the P-38 did not have as much success over Germany. In this northern climate, it was an "ice-box" on high-altitude missions, and it was out-performed by the Fw 190 and the later marks of the Bf 109. The main reason for the P-38's relative failure in high-altitude operations in the European Theatre was due to engine failures experienced above 20,000 feet. Frequent engine failures were attributed to failing spark plugs and other parts that could not use the European, rain-drenched fuel. Many of the aircraft problems were addressed by the P-38J variant, but by September 1944, all the Lightning groups in the 8th Air Force had converted to the P-51. The Eighth did continue to operate the F-5 recon variants with more success.
The P-38's service record shows mixed results. On the negative side, most variants were certainly harder to fly than the best single-engine fighters and in early models, pilots suffered badly from the cold in northern climates. Also, the twin turbocharged Allisons had problems – a good portion of Lightnings were lost during the war due to engine difficulties rather than by enemy gunfire, which contributed to the plane's relatively low kill-ratio. Up until the "J-25" variant, P-38s were often "sitting ducks" to Luftwaffe fighters because of the problematic engines and the lack of dive flaps to counter compressibility in dives. German fighter pilots would often go into steep dives because they knew that the Lightnings would be reluctant to follow.<<
Once they changed the engine on the Mustang, it was a terror.