You know its a tough call. Jack Ilfry thought the P-51 was a better escort fighter, and in fact that was the only place the P-51 shined. Performance wise
P-51D - best climb is at 5,000ft - 3,320ft/min (7.5 min to 20,000ft)
P-38J - best climb at SL - 4,000ft ave climb to 20,000ft - 3,724ft/min I have a P-38L graph that matches this 5.37min to 20,000ft) 'Planes and Pilots of WWII' site. An AAF test puts it at 4.91min to 20,000ft, I don't have it so Its just a possible at this time.
P-51D - Top speed - 443mph
P-38L - Top Speed - 442mph (I have a Lockheed graph and have been told there is an AAF test confirming this) however I think until more solid data surfaces (the AAF report) I'll go with 431mph.
P-51D - Acceleration - 2.2mph/sec @ 15,000ft from cruise
P-38L - Acceleration - 2.8mph/sec @ 15,000ft from cruise
P-51D - range - 2,200mi
P-38L - range - 2,300 mile mission carried off in early '45 (see Smithsonian site)
P-51D - gun package - 6 .50 M2 guns - sited at 250yards
P-38L - gun package - 4 .50 M2 guns, 1 cannon 20mm - effective to 1,000yrds.
P-51D - load - 2,000lbs
P-38L - load - 4,000lbs - 5600lbs reported from the field.
Maneuvering capability. the AAF considered the F model as good from 10,000 to 15,000ft and the P-38F better that the P-40, P-47, P-39, and P-51(the best handling Mustang model) above 15,000ft.The L was reported better, and with maneuvering flaps deployed was reputed to be able to turn with anything, check the John Tilley story where he out turned a Zero at 90mph and 1,000ft. The L model had compressability slats deployable at any speed and could momentarily pop the nose up 20deg at any speed, and could dive after anything. With the aleiron boost could out roll anything above 350mph (I can post a graph) or its available at the 'Planes and Pilots of WWII' site.
The P-38 out performs the P-51D in all but top speed, and cruise speed, and cost. Cost was not an issue durring the war, they made ~18,000 P-47s which ranged from $500 less to ~$8,000 less and wasn't up to the job until the D model was introduced.
wmaxt