Dotsie's right - the UU cover's impact is subtle and as it's evolved (with the aging treatment) artistically
, it's a sophisticated statement about the story and it's translation to Discworld. To put a US slant to it, it's like an original amateur photograph of Babe Ruth taken making some crucial home run, from the perfect angle then slapped in someone's photo album and left in a garden shed for several years then discovered years later and put to auction for top dollar.
Paul's original artwork was too pristine and didn't work for that reason - the design team's aging of the 'paper' and making it look like an ancient sepiaed iconograph is the key element and that's what makes it brilliant.
The Thud cover is definitely the better of the two skill-wise but the symbology and 'rightness' of the UU one is the more cerebrally successful