I'd say they rank about equal for me, Tsubasa is huge and a bit bewildering at times but I love the epic feel of it- and I also love xxxHolic's smaller setting, and Yuuko of course. IMO it's just not the same to read one story without the other, they're both great- maybe Tsubasa's got a bit repetitive in early parts, but it really picked up after the Rekord Country arc.
xxxHolic is everything it is because of Yuuko. She is absolutely my girl crush..