Actually, if you're talking about Black and White, Zekrom is in White and Reshiram is in Black.
In Black and White 2, Zekrom can be caught in Black 2 and Reshiram can be caught in White 2. (Along with Black or White Kyrum, respectively).
I personally think Zekrom looks cooler, but I like White Kyrum more than Black Kyrum, that's why I have White 2. (Plus cooler exclusives, and White Forrest > Black City).