Dungeon Explorer II is much better than the first one, and I say that as a fan of the first one. The quest is much longer, there are way more bosses and characters, the story and cinemas rule, and the soundtrack is ridiculously good. Personally, I'd say go for the US version because you won't get to experience the true awesomeness of the villain if you can't understand what he's saying. But if it costs too much these days, then yeah, go for the JP one.