I feel that Raikou is the kind of 'mon that brings very tense weather with him wherever he goes. A thick cloud layer, heat and humidity. A faint rumbling from distant thunder, a few drops of rain. Where Raikou walks, it's always at the brink of a thunderstorm. That is, as long as it stays calm and collected. Once something angers Raikou, lightning strikes and the winds start howling. Raikou certainly is a loaded spring, a bomb waiting to go off. It looks like it can wreak havoc any moment it wishes, and you really shouldn't give it a reason to.
But Zapdos, on the other hand, has only one mode: Thunderstorm. Whereas Raikou embodies the silence before the storm, Zapdos is the storm. Clear skies are covered with black clouds from horizon to horizon in mere seconds when Zapdos appears. Thunderbolts fly everywhere, lightning strikes and the wind whips up large waves. You'll rarely ever see Zapdos, only its spiked outline through the clouds whenever an especially powerful thunderbolt illuminates the storm. Yes, you can feel Raikou's precense and (wisely) decide to stay hidden, praying that you don't piss of the majestic tiger, but you can see Zapdos coming from miles away, bringing with it the worst weather you'll ever witness. You can't stop it, you can't change its direction. When Zapdos is out flying, you better secure your belongings and go hide.
And should Zapdos ever meet Raikou... Raikou might roar at Zapdos, being among the extremely few who dare to stand up to it, but Zapdos is clearly the superior electric legendary. It is the embodiment of a thunderstorm, and it behaves every bit like so.
