I've used both and I think that CKEditor has more advantages, extensions, demos and options (for developers).
I know that WordPress uses TinyMCE and it may be hard to switch to CKEditor but it wouldn't be a bad move in my opinion. Worth discussing.
TinyMCE bug #1: color the text with green and color half of the green text in red. Result: it nests <span> with red in <span> with green which isn't correct if there is a simpler solution (i.e. don't nest <span> in <span> and CKEditor does it right). Correct: http://ckeditor.com/demo#full
TinyMCE bug #2 (serious): not possible to create nested lists? Why "indent list" creates blockquote if there is another "blockquote" icon available?
TinyMCE has just many little and annoying issues here and there while CKEditor has none serious. With TinyMCE it's not difficult to end up with messed and INCORRECT HTML and a post/page that can't be rescued. It just doesn't work right.