There's been a lot of talking about a Sun report claiming the xml performance in Java is better, compared to the performance in .Net. While the performance of the xml implementation could be better in some instances, serialization for example and xslt, in most cases caching can reduce a lot of the issues.

Mark Fussel has some other interesting points on ease of development with the .net classes and the performance of the v2 (Whidbey) implementation.