In my opinion, xxxHOLiC is definitely superior to Tsubasa. As I've said before, I feel Tsubasa is a collection of cheap knockoffs of CLAMP past characters + action.
xxxHOLiC actually has something different and innovative going on with its art, being a modern interpretation of ukiyo-e. This leads it to be more beautiful and the clothes are more interesting, in my opinion. Because they are all original, Yuuko, Watanuki, Doumeki & co are a lot more interesting to me than all this Syaoran/F!Syaoran nonsense.
But that's just me.