I think model with relatively easy main game and hard side quests is the best. main game should be playable for more of people, even for those that haven´t much experience with JRPG (or any genre, after all). This way, everyone can play.
For more experienced gamers, there should be some difficult side quest, like hard dungeons, monsters, and so. Or, some harder difficulty as whole (Like SO, most Atlus RPG, etc...). SMT: Nocturne on hard definitely wasn´t easy.
IMO, games like FFX are perfect example of good difficulty. Game can be finished with some effort, maybe one, or two deaths, when some boss make weird move (Yunalesca and Megadeath says "HI"), but it´s bearable. On the other hand, there lot of side quest bosses, like Dark Aeons, Penance and such, which are pretty hard and your chars must be in best condition. From difficulty view, perfect game.
