It has been observed by an experienced JDM importer that USS auction is a bit generous in their grading, what you may consider a 3 would likely get a 3.5 and the same goes for the interior. The *AA auctions tend to be more strict with grading so a graded 2.5 is probably a 3. Interior, same thing.
You tend to get better prices at the *AA auctions as well but for volume USS auction cannot be beat.