Game7 writes about the Graphical differences between Final Fantasy XIII and the nearly two year old Lost Odyssey. There are some Screenshot comparision for Character-Detail, Monsters and Architecture, since the Games can not be compared 1:1.
Translates Qoutes:
"Lost Odyssey looks still good in this comparison"
"Final Fantasy XIII has quite bald extensive areas, but usually more detailed background. The characters are very detailed in both Games and here your personal taste decide. The finelined more girly characters from Final Fantasy, or the rougher more western-style Characters from Lost Odyssey"
Look at the results for yourself...