There are, roughly speaking, two types of generals: maneuvering generals and attrition generals.
The difference between the two is how they even the odds. Maneuvering generals use superior mobility, tactics, and aggressiveness to confuse, surprise, isolate, or surround their enemies. Maneuvering generals recognize that an enemy who's resolve is broken and/or is on the run, is ripe for the slaughter.
Attrition generals use terrain, unit effectiveness/firepower, guerrilla/Fabian tactics, alliances, economic/morale warfare and grand strategy to nullify overt advantages like numbers and force battles in contexts that favor them. Attrition generals shine when faced with armies that are either larger or led by famous and therefore intimidating generals.
With this in mind, it's obvious between Napoleon and Wellington which one is which. To further clarify, maneuvering generals are superb field commanders, whereas attrition generals are better strategists. This is why Scipio Africanus beat Hannibal, despite Hannibal's clear superiority in the field.
On topic now, the only way Wellington could lose in Spain is if he:
1) Blundered away his troops
2) Either outran or got his supply lines cut (either through loss of control of the seas (Thanks Nelson!) or loss of home support)
3) Allowed the Spaniards to be beaten and cowed.
With this in mind, his strategy and fighting style became obvious, but credit must be given as the Peninsular campaign was a tour de force of attrition strategy. The Spanish guerrillas (funded and armed in part by Wellington) were much more effective than the Spanish regulars, and successfully defending Portugal (repeatedly!) as his supply base were masterstrokes. And kudos to Sharpe, for both introducing me to Wellington in Spain, and being an all-around badass.
Napoleon lost Waterloo because he failed to isolate and destroy the weaker force, which was Blucher's. If he had feinted at Wellington (who was busy hunkering down on the reverse slope) and pushed Blucher further away from Wellington, he could have forced Blucher north-east and flanked the escarpment at Waterloo. Wellington, in turn would be forced to retreat to protect Brussels (or else have Napoleon threatening his rear and his lines of communication) and Napoleon may have had an opportunity to hit Wellington in the flank and isolated from Blucher.
But Napoleon had never fought the British (and thus failing to recognize Wellington's skills on the defensive and dealing with flighty allies, ditto the superiority of British musketry). He assumed that between Wellington's hodge-podge army of (to him) debatable quality, and Blucher's Prussians, that Wellington was the weaker of the two, despite being the larger force. In reality, placed on good defensive ground and with allies to support, Wellington was much more dangerous.
Napoleon also lost in Russia because Kutuzov and Barclay de Tolly were also attrition Generals. He over-concentrated his Grande Armee - stressing his supply lines and foragers to their breaking point, then got seduced into a classic attrition battle at Borodino, and was successfully hoodwinked by Alexander I into believing he was ready and willing to make peace. This left him hundreds of miles into enemy territory, with the Russian army and government still intact, and winter coming, compounded by Alexander's stalling, which he played into by staying in Moscow. The space of Russia and the Russian generals' attrition tactics nullified his maneuvering and tactical advantages and he mistakenly identified the Russian center of gravity as Moscow, rather than the Czar himself, safe in St. Petersburg.
Wellington by both experience and inclination would not (and probably could not) have succeeded where Napoleon did, whereas Napoleon had some understanding of attrition tactics (just not how to defeat them). So therefore I give the edge to Napoleon, while recognizing that Wellington was just the man to beat him.
However, I think Caesar puts them both to shame, demonstrating successfully both attrition and maneuvering strategies in his campaigns to great effect.