Pollock is a better bowler than Kallis is batsman. Pollock's average of 21 for 300+ wickets means he is quite possibly the best bowler of this era (yes, I know a lot of you won't like this). Kallis on the other hand is a world class batsman, but he is clearly behind Tendulkar, Lara, and quite possibly Hayden, Dravid, and Ponting. However, I think Kallis' bowling, with an average around 30, is slightly better than Pollock's batting, with an average around 34. Kallis' batting average is 20 runs higher than Pollock's, and Pollock's bowling average is 10 runs lower, so I give it narrowly to Kallis (and yes, I realise that working out the differences in averages is a flawed method of determining this).
Also, what about Lance Klusener? I dunno about his test figures, but is he the best ODI all-rounder ever?? He must have taken a host of wickets, and his batting record is phenomenal-averaging 42 and striking at a run a ball.