Let's start with Blue Dragon for the Xbox360:
1. Bland visuals. Although the character models are excellent, the background and environment graphics are dull and bland. Characters' costumes are plain too. Not something you would expect for a multi-disc next-gen game.
2. Battles can be boring. Blue Dragon uses the standard old-school turn-based battle system. Many J-RPGs have tried to spice up the battle system, as shown in every Final Fantasy games, but Blue Dragon returns to old-school. Result? Kinda boring. There is no sense of urgency (unless you want to escape from battle). Also, the characters are mute during battle and victory sequence.
Now, Eternal Sonata for the Xbox360:
1. Beautiful and amazing visuals. Very colorful, kinda like a more cheerful version of Folklore. Character costumes are very detailed.
2. At first, the battle system looks like a standard turn-based system with AP points. You can move freely around the screen and do attacks until you run out of the AP meter, and the meter only drops when you move/do an attack. Sounds easy huh. Well, not too far into the game, the party level will go up, and it becomes interesting. At Level 2, the AP meter will also act as a timer. Once you start moving the character/doing an attack, it will deplete continuously, putting a sense of urgency to select attacks quickly because once the AP meter runs out, the character's turn is over. At later levels, the battles will become more like real time. It's more engaging and fun (although not as fun as Grandia II). Even better, the characters are fully voiced during battle, yelling special moves and have cute/cool victory sequences.
In short, if you're into old-school RPG, and like stats/class systems/monotonous leveling, then get Blue Dragon. However, for better visuals, more interesting story/characters, and fun battle system, get Eternal Sonata. Of course, you can always get both like me. Waiting to see if Lost Odyssey is worthy of the big names of Mistwalker.