Compare TinyMCE and CKeditor for a Wiki
I spent some time implementing CKEditor in the last couple days. I've implemented TinyMCE in the past as well. On the positive, it's far more consistent and bug-free than TinyMCE... by which I mean, where TinyMCE "feels" buggy, CKEditor has worked around awkward browser behavior to a much greater degree, making it "feel" much more solid. On the negative, if you want to extend it, the documentation is relatively sparse. I think this is mostly because CKEditor is relatively new (its API is very different from FCKEditor), and it would be reasonable to expect the CK 3.0 documentation to reach at least the quality of the FCK 2.0 docs soon.
I've been using both editors since some years ago... Almost always I've chosen CKeditor over TinyMCE.
The reason?
Short answer:
CKEditor is very stable and very easy to use and has integrated the file manager (with an ad, but it is no problem for me), but TinyCE has not any integrated File manager.
Nevertheless, I like JCE editor (for Joomla), this editor is based on TinyMCE and works like a charm. It has a very good implementation of File management.
If you plan to use a WYSIWYG editor for a wiki, any of them are ok, because you don't need a filemanager (I think).
However, I recommend you, based in my experience, CKeditor.
The long answer is very long for this space. If you want the long answer, contact me or google around about this topic.
A cople of other Wysiwyg editors
http://imperavi.com/redactor/ (paid - actively developed)
http://xinha.webfactional.com/ (updated 2010)
http://www.kevinroth.com/rte/ (updated 2010)
http://nicedit.com/ (updated 2008 - small fix 2012)