Okay, as suspected the xanga problem was more a problem with xanga.com than with the clients I tried. If you have a xanga.com account and want to use it with a blogging client, you can find instructions to do so, here.
So for me, it has come down to a choice between Zoundry Raven and Windows Live Writer and of the two Zoundry Raven would win, if I could get it actually do what it says it can do and post to three different blogs simultaneously without error.
Windows Live Writer has the tiny drawback that you need to change the weblog you want to post to, manually, but it does keep the post and just changes the format to suit, so you don’t have to retype the entry multiple times. Tagging for livejournal with it is a bit clunky, but not a deal breaker for me.
Posted with Windows Live Writer