Played about halfway through both of them, and love the stories for both.
I have to say that even though DQIX is one big grind, it dosn't feel like one, and even though ToI isn't much of a grind, it feels like one.. probably because of a lack of character customization? I don't know. Battles feel more strategic in DQIX as well.. battles in ToI kind of feel like punching a punching bag to death over and over.
I have to give full props for the translation work in ToI though, better than most professional translations. The character interactions in ToI really make the game for me, the amount of party banter and events keep you in the game more than the gameplay does. Character interaction is a non-issue in DQ!X, so in a way these games are polar oppisites in what make them good. Its hard to say clear cut which is better, because they are both strong in different area's.