Not having a horse in this race, as I like these two fighters just about equally....
From Dean's
America's Hundred-Thousand:
Take-off distance (gross weight/zero wind/hard surface/sea level/take-off power):
P-38J
1,080ft at 17,699lbs
P-51D
1,185ft at 10,176lbs
Power loadings (gross weight/horsepower) at various altitudes, when compared to 11 other fighters:
P-38L ranked
first at 10,000ft/20,000ft/30,000ft
P-51D ranked
2nd at 10,000ft and
3rd at 20,000ft.
The P-38L ranked
first in level flight acceleration when compared to seven other late war US fighters. The P-51D ranked
third (when starting at 250mph and applying combat power).
The climb rate of the P-38J/L in NORMAL power is shown to be
superior to the P-51D in either NORMAL or MILITARY power, at practically every altitude.
Speed, range, roll and turn rate go to the P-51.
In addition to this, the author makes mention of the Joint Fighter Conference held during October 1944. Out of all the categories where both the P-38L and P-51D are mentioned, there are four where the Lightning is ranked higher than the Mustang by sheer number of votes:
1) Best overload takeoff from small area (something we discussed earlier).
2) Best rudder.
3) Best characteristics 5 mph above a stall
4) Worst cockpit (obviously not a positive trait at all, for which it ranked 1st)
In contrast the P-51D ranked higher than the P-38L in nine categories (when both are mentioned), such as best engine controls arrangement (1st), best ailerons at 100mph, best ailerons at 350 mph (1st), nicest all-around stability, best elevators, best dive stability and control (this one is easy to understand), best all-around fighter above 25,000ft, best fighter-bomber, and best strafer.
There is a strong possibility that the majority of the pilots present at the conference weren't twin-engine qualified, which obviously would make the P-38 a handful to operate. But without knowing the backgrounds of these pilots this is pure speculation on my part.