I feel Vesperia uses a style and technical level that they were more capable of working with.
Xillia tries some things that have greater potential than Vesperia in terms of scope and detail, but I feel that overall its visuals aren't as polished and consistent in regards to art and how it affects performance. The next game will probably (hopefully) be better, but Xillia has a lot of rough patches.
No Tales game is really going to be a showcase, so it helps a lot if they use an appealing style with a good level of consistency throughout the game. Aside from things like animations, Vesperia's visuals may end up having better longevity.
Yeah. Honestly, Xillia would be pretty nice if they were just consistent. I liked the main characters' models and movements and I loved how pretty the first city was. Everything else looked like a nicer Abyss or something.
Regardless, it was still a nice graphical leap for a non cel-shaded Tales. Even if Vesperia looked nicer.