If you are going to consider an AT XI, I would have to go for Sobers over Imran, although I believe that in general Sobers is overrated and Imran is underrated.
For one, as has already been pointed out, Sobers's batting > Imran's bowling and Sobers's bowling > Imran's batting.
Regarding the bowling, Sobers had a freakishly low economy rate (2.2), so using him in an AT XI scenario would force the opposing batsmen to try and score runs off Akram, Marshall, McGrath, Hadlee, Warne et al, which would be suicidal. Plus Sobers had the ability to bowl more than one style, which means that he would be more versatile.