Actually Captain Boyd Wagner was a USAAC P-40 ace from the Phillipines and said the P-39 was the better fighter, he scored kills in both types.
First P-39 vs A6M2 Zero encounter was a dogfight over Lae, New Guinea, P-39: 4 kills, Zero: 3 kills.
It was much more the tactics employed than the a/c. Neither the P-40 or the P-39 were veiwed as a match in a turning fight vs IJ fighters, and the crews wanted a fighter that possessed better speed and survivability, which is why the P-38 and P-47 replaced both as the front line USAAF fighter in the Pacific in 1943.
...and Russian air force units did not count ground kills. The Russians flew some of the later models of the P-39 like the N/Q version, and their main job was shooting down bombers and recon a/c, not just enemy fighters. On the Eastern front, the P-39s short range was not a hindrance, as most VVS units flew close to the front, and most combats took place below 15,000 ft, so they were able to get the most out of it. It was never an uber fighter like the LA-7, or Yak-9U, but it served well. The Russians genuinely liked it, despite it being a "foreign" type.
There is a lot of "bumper sticker" crap said and written about the P-39, some of it deserving, some not as much.