I started using Akeeba Remote Control for Joomla and was really impressed with it. When I went to look for something similar for WordPress, I found these Weblog clients.
Which one is the best for backing/restoring up your content?
(Existing and content that you add to the blog via the Weblog client)
It is not really direct answer to your question, but I think complete (database plus files) backup/restore is more comprehensive and important.
Content backup is only content. Database is all of your blog, which can help you go from nuked database back to normal in no time (speaking from sad practical experience, that would be much more sad without complete database backup).
Would you like to keep your question about clients alone or expand it to backup/resotre in general (it’s marked as community wiki so I am trying to establish scope) ?
Windows live writer is best for this. You can download it freely from mss site. It supports draft and xml rpm. Dont forget to enable it from dashboard