http://www.xara.com/us/web-designer/whats-new/ A comparison.

In WD10premium there is Xara's version of responsive design [variant sizes for mobiles etc without having a different URL] for one thing. Try the 30 day free trial Yes, premium has more than WD10 basic.

You don't have to remove old versions, on the other hand they are not needed for any new version to work but if you want to uninstall old versions I'd suggest you do this before installing the new.