Our reasons we use CKEDITOR:

A. We have not had any of the issues we had with RAD (empty file / image lists, etc.)
B. CKEditor is way better then RAD at handling text pasted from rich-text environments (like word).
No need for paste as plain text any more..
C. CKEditor is more like FCK (less to explain to upgraded clients)
D. The developer of the CKEDITOR provider fo rDNN (Ingo Herbote) is one of the most active community members, if you find an issue and log it on Codeplex it's mostly solved within a few days.
E. CKEditor is one of the "standard" WYSIWYG editors on the web, so it's used way more then RAD.
This means more testers and thus potentially less issues.
Plus a bigger chance a new editor already know the editor from other non DNN environments

Advantage of RAD is the central settings editor.
For CK you do this on the editor itself (per portal / page / module), but you have a little less overview.
But it does mean the admin has access to these settings

http://dnnckeditor.codeplex.com/ 

HTH