I used GMVault only because Google's solution was not out there at that time. However, as others have mentioned, GMVault works on an incremental basis after the first download, while the Google solution seems to be a snapshot-only service (not 100% sure yet).

By the way, my first download with GMVault (eml format, each email separately gzipped up because I did not read the command line options properly) was : 22900 mails which took up 2 GB of disk space.