<p>I have to choose an online WYSIWYG editor. I'm pending between <a href="http://tinymce.moxiecode.com/" rel="nofollow noreferrer">TinyMCE</a> and <a href="http://trac.xinha.org/" rel="nofollow noreferrer">Xinha</a>. My application is developed in Asp.Net 3.5. Could you help me with with some pros and cons? </p>

I've never used Xinha, but I can vouch for TinyMCE. It's fast, scales well, and is infinitely customizable. I particularly like the dynamic loading of functionality, which means you only take the performance hit for the stuff you use.
It also includes language-specific
compressors
to further increase performance (C# is supported, along with PHP, Java and ColdFusion) by GZipping components.