Bah, I'm getting close to the end of HeartGold and only just discovered this thread lol. Oh well, there are massive Pokémon forums about the place so I didn't expect much discussion here. After the Johto nostalgia wore off, I realized I still like Platinum more. FireRed too for that matter - the Sevii Islands are still my favourite part of any game.
The reason is that Johto is fundamentally lacking - even on the DS. Its so linear and one-note, and Kanto is still watered down compared to when its a stand-alone region. They tried to add in some new stuff like Battle Frontier and whatnot, but those are just specific locations. The overall effect is the same - Sinnoh still seems way bigger/more interesting than Johto. And even though Hoenn was annoying with all the water, at least it had a lot of environmental variation, which kept things interesting. HGSS was still quite enjoyable though, just because its Pokémon and you really can't go wrong with a formula thats sold almost 200 mil games.