xxxHolic is maybe my fav manga, strangely enough I haven't read Tsubasa Chronicles yet. In xxxHolic you can find characters from other Clamp works like Cardcaptor Sakura (you can say most of the CCS distribution: Sakura & Syaoran, Toya, Yukito, Clow Reed, Fujitaka, Kero-chan) and Chobits (Chii is so sweet).
The surprise is the twist of the story. In the beginning of both xxxHolic and Tsubasa Chronicles is mentioned that there are many worlds inhabited by versions of the same people which share the same soul (at least this is what I understood, since they met the same people with the same names and even relationships in different worlds). But in the latest chapters of TC (I know this because I'm a spoiler addict) it is revealed that the characters from CCS and TS are actually connected, so in fact xxxHolic and TS are sequels of CCS (I won't give any more spoilers, I know from my own experience that they only ruin the story and this is one of the best so it's better to read and enjoy).
In conclusion if you want to read something original and unpredictable for a change read xxxHolic.

[color=red]PS:[/color] Neither one (xxxHolic or TS) are side stories. Why would anyone say that, it's plain stupid. First: each of them have different main charactes. Second: any of this stories could exist without the other, untill now I have only read xxxHolic and it made perfect sense to me, instead of the TS cross they could invented another story that could be as good. The only reason they made this is because it's original and because their previous mangas had great success, this way insuring the sales of this ones. They need fans to buy their works don't they?

[img]http://img91.imageshack.us/img91/120/zzzkq5.jpg[/img]

I love [color=red]vampires[/color].

Okay, who has read Tsubasa chapter 201? If you haven't do now. It basically explains the nature of Watanaki's existance, the whole ying/yang reference Yuuko made in early chapters and his connection to Sayaron!!! Seriously some things are becoming clear!!!