Both were at Southside Festival (Neuhausen ob Eck, Germany). Lily Allen gave a mildly uninteresting concert, I listened from a distance while exploring the site. Katy Perry, on the other hand, completely messed up. Started 3/4 of an hour late. Everybody was just sitting on the lawn and relaxing, many just laughing at her. Quite the epic fail.
Also, Lily Allen appears to be human, and probably quite a pleasant person. Katy Perry - not quite, IMHO