I am in the camp that it wasn't really a case of either, but a combination of the two.

WWII, like most all wars, is a complex tapestry of events that isn't defined by one or two acts. WWII, given the sheer scale and scope of the war, is especially complex.

The thing is, mistakes by one side (Axis losing) mean very little if the other side doesn't capitalize on them (Allies winning). Mistakes may make it much easier for one side to win, but again we're back to square one with the Allies being able to claim they won by taking advantage of such mistakes. To give a specific example, did the Germans lose by virtue of not having a proper long range heavy strategic bomber, or did the Allies win by virtue of having one (or rather many)? (Please note, I'm not in any way whatsoever suggesting that that factor alone cost Germany the war or won it for the Allies. Again, it's way more complicated than that, and there wasn't one magic bullet that Germany, or the Axis as a whole for that matter, was lacking that could have won them the war had they had it.)