[Omicron]

I think Raikou's main draw over Zapdos is its higher Speed. Base 115 Speed outruns quite a lot in the Dream World metagame, and most Pokemon are unable to outspeed it without the aid of a Choice Scarf or some other sort of Speed boost. Raikou has only one weakness when compared to Zapdos's two, and although it has a Toxic Spikes / Spikes vulnerability, it doesn't have a weakness to Stealth Rock, which can severely limit the number of times that Zapdos can switch in, and also lowers its durability. Zapdos also has no reliable way of boosting its Special Attack except if hit by an Electric-type attack to activate Lightningrod. Raikou can easily find opportunities to boost up with Calm Mind and hide behind a Substitute that cannot be broken by most special attacks after a few boosts. Zapdos may have reliable recovery in order to combat Stealth Rock damage and the like, but I feel that in general, its lower Speed and more weaknesses make it a slightly inferior choice to Raikou.