That's unfortunate and my fault, basically, the .tgz tarball was built with a BSD-ish tar while the .xz one was built with a GNU tar, as intended.  I hate to do a stealth update, unless absolutely necessary, and I don't get see any errors unpacking the .tgz one with a recent GNU tar.  Can you say with which version of tar and on what platform you see the tar failure?  I'll make sure it doesn't happen in subsequent releases.