Oracle 11g XMLType Experience
I have worked with Oracle Xmltype for the last 4 years, starting with Oracle 10.2.0.4 and now on Oracle 12c. I find it very reliable. We have used it not only for storage, but also XML manipulation. In that regard Oracle has chosen XQuery above XSLT.

The performance is dependent on the way you define the storage of these XMLs. You can store them as CLOB, as object-relational storage or (in 12c) as binary XML. The choice that you make depends on different parameters. CLOB keeps the white spaces in the XML. Object-relational storage is the smallest and binary XML (the default in 12c) is somewhere in between.

Regarding Query speed, I prefer binary xml, but all 3 solutions are very much workable in my experience. I didn't see any performance differencies between CLOB and Xmltype (stored as clob). When the XMLs are stored in a more compressed format, you could even gain speed.