Both are very good editors and I've used each one in other web applications. The reasons for FCKeditor are sound and I have found it a fairly easy to customize. It does appear to have more active development than TinyMCE when you review the current features and updates.
I was able to add a third row of buttons. I added such things as the Paste from Word, Show Blocks, insert quote, search and replace, spellchecker, special characters, smileys, styles, insert rule and select all. All of these features appear to function fine. I understand the desire to maintain simplicity and keep the toolbar as uncluttered as possible, but these are helpful features.
It works well within Safari, except for the context menu. It is partially off the bottom of the browser screen and pretty much unuseable within Safari. The problem does not exist in Firefox on the Mac.