In my pure state, well in my own taste, I like xxxholic better. Because Tsubasa is a format that has already been done. Not that I don't like it, but traveling to different worlds with companions on a epic quest is something we have all seen once or twice in anime or manga.
xxxholic has some great character developpement in my opinion and some darker themes that sometimes reminds me of Tokyo Babylon. Yuko-san is an amazingly cool character and her morals and ways are fascinating.
I wished that xxxholic would have been made into a series and Tsubasa as a movie, not the other way around.