I think Astraweb and Blocknews are probably very similar when it comes to posting.
Keep in mind that posting to usenet is not anonymous. If someone really wants to track down an uploader, they can do it because ALL providers log upload details such as the user's IP address to fight spam abuse. I've never known anyone to be sued for posting a copyrighted file, but I've known several people (using various providers) who got their upload ability blocked.
USPs are supposed to restrict the upload privileges (or even cancel the entire account) of habitual copyright infringers (Giganews reportedly does this a lot) so if you do a lot of posting from a block account, it would be wise to use one that is nearly empty of download bytes, just in case it might get canceled.