RadWordsProcessing is intended as a library completely independent from UI. For this reason it doesn't provide UI and we have no plans to introduce such functionality. If you need to show and edit documents, RadRichTextEditor is the way to go.
The two components don't share a common code base, although they both use some utility methods and classes. As to the quality of the import-export roundtrip you want to achieve, this greatly depends on the exact content that your documents have. WordsProcessing is fairly new, so it still misses some features although we are actively working on implementing them. Also, it's a bit easier for us to add new features to it, since there is no need to implement them in the UI. RichTextEditor, on the other hand, is almost feature-complete but has some limitations in terms of HTML support
My suggestion to you would be to try them both and see which one fits your needs better. If you have any questions or concerns, do not hesitate to get back to us.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.