Naturalization vs. Industrialization, and here's why:
Zekrom's tail is like a part of an airplane (sorry, don't know the correct term) while Reshiram's liiks like an old style torch. Likewise, Zekrom's typing is electricity, which is what we as humans use today in our modern civilizations for light, warmth, and several other things in our daily lives. Reshiram is a fire type, the source of light and warmth humans used before electricity was harnessed for daily use. In addition to that, Zekrom is very sleek and artificial looking, while Reshiram is feathery and looks more like a natural animal.
The version exclusive areas are Black City (industrialization) and White Forest (naturalization)
And why do these legendaries have to "top Arceus"? Maybe they can still be amazing without being overly powerful or important towards existence?