>Kanto
Decent city layout, with a central hub in Saffron that connects the majority of the region. Low geographic diversity, with the only thing breaking up the forests, grasslands, and caves being a few abandoned man-made structures (which, to be fair, makes it a decent choice for urban explorers). Probably has the typical humid subtropical climate that one would expect of Japan. Biodiversity is decent enough, but with a huge bias toward poison types, which would make the local wildlife particularly dangerous. Poor infrastructure within smaller towns makes visiting places like Pewter a dangerous and difficult prospect. Only area with consistently cold weather is a remote island, so this is a poor choice for those who prefer colder climates. Also a poor choice for those looking for an island based home, as the only feasible choice (Cinnabar) blows the fuck up. Entertainment is decent, at least, with things like Game Corner, the Cycling Road, and Safari Zone being places of note. Easy access to Johto.
>Johto
Not as urban as Kanto, with Goldenrod not being as central as Kanto's Saffron, and as such getting around the region is even more tedious than it would be with Kanto. Slightly more geographic diversity than Kanto, thanks to its Ice Cave being more easily accessed. More natural landmarks of note, including an extensive, region spanning cave system, a massive lake, and an island town that doesn't blow up. Decent biodiversity, though a shocking lack of Dark types for some reason, making this a poor choice for those who prefer that type. Fantastic cultural heritage, with several landmarks spread across the region. Maintains a very homely atmosphere as a result. Many options for entertainment, with a Game Corner, National Park, several historic buildings, the Dance Hall, and a Battle Frontier. Also has a Safari Zone, though it's far more remote than Kanto's. Easy access to Kanto.
Might post more autism later.