This is a hard question. Spitfire was available from 1939, and was competitive throughout the war. it was not much of a ground attack aircraft either, with some unforgiving flight characteristics. As a carrier plane, its debut was pretty por, but by 1945 it was the best CAP fighter of either Navy, and with one of the lowest operational loss rates in the BPF, and indeed for both Navies. By 1945, however the decisive carrier fights had been won. In terms of "stretchability", the Spifire was capable of more development, and remained in service as a frontline aircraft for a lot longer
Hellcat was just what was needed in the midwar period. it was tough, fairly fast, heavily armed, and protected, good at carrier operations, possessed good range and made a meal out of it enemies, though not the 19:1 being claimed (they are the claims made for it, and especially annoying is the totally spurious claim that it achieved 19:1 against the zero...it was more like 4:1 against the zero in the air...still good, but not the fantastic ans spurious claims that are so often t4rotted out to promote it as some kind of uber fighter) .....nevertheless its combat record is undoubtedly very high....and it wasnt against an enemy that was second rate......it did well against the Germans when ranged against them as well).
I think you have to give a qualified answer here. For the Pacific, its the Hellcat....availability, the right package, an effective combat sysatem. In Europe, where carrier operations in the last half of the war were not as important, the Spit was probably more valuable. It contributed to the early defensive victories, and remained competititve against the opposition to the end.