Johto
>most routes look the same
>small region
>a lot of dead space where more trainers or treasures could be included, routes are big but with few things you can interact with
>has dead ends, you can only reach locations from the same angle every time
>all the region is flat
>only 10 towns, with just one being a proper city, which adds to the feeling of repetitiveness
>most areas are not worth revisiting, so you end up ignoring most of the map once clearing
>has battle tower

Hoenn
>every route looks drastically different, varied terrains and layouts
>huge region, at least double the size of Johto without even counting the water routes
>filled to the brim with varied encounters at every corner that make every route feel even more unique, no space is wasted
>everything is looped and interconnected, so travelling around stays fresh
>has many mountains and valleys that you can explore
>15 towns, many villages and cities
>many places worth revisiting: contest facilities in many towns, daycare at the center of the map, secret bases
>has battle frontier

Even if you play gen 2 for the Kanto section, you're better of playing gen 1 directly instead of the demaked version for babies who get scared of the Lavender theme.