2
line breaks render differently based on a lot of things, not just the WYSIWYG editor - I find using Firefox gives me the best results with any WYSIWYG editor. Personally, I like Koivi for the fact it is fast and I can integrate the system imagemanager with it. So the same images are available if you use the DHTML editor or Koivi.
For a comparison - start with
this threadIf you want an better DHTML editor, installing the xoopseditors class gives you Extended DHTML - a few more options and you can highlight text and click the formatting button to change the format. It also has some features like being able to use wiki links, resizing images, and shortening URLs.