From: ZFS | #032
FFXII is more open-ended all around than FFXIII. There is still a linear story path to follow, but they're very clearly different ideas.
It's more open-ended in the sense that you have a lot more empty space to run around on. I suppose the main difference is being able to do some sidequests early.
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This is what SuperNiceDog did to me in the Guru: http://i43.tinypic.com/dgwh0.gif
http://www.majhost.com/gallery/atukam/OHWD/snownd.gif