In Test cricket against Test standard sides, purely as batsmen (who on Earth is comparing them as bowlers???) Tendulkar's career is infinitely more the noteworthy.
Tendulkar, between becoming Test-class (I think we can forgive him for not being so immediately he was selected, as he was only 16) and his decline, which began in 2003, averaged 61.20. This is absolutely awesome. In this time he displayed pure excellence at bashing rubbish attacks, and also played any number of superlative innings against the better ones.
Unfortunately, he's not been the player he once was for the last 4 years, though of late he's been better again if not exactly at his previous level. He was still far from poor, averaging 42.03. But this disguised the greater pattern - first it was the occasional massive score offset by loads of failures, then later on far too 20s and 30s and not enough 60s and 70s.
Kallis, on the other hand, also had an awful start to his career. When we take his arrival as a true Test player to the time of run-scoring becoming that much easier, he was a very fine batsman, though not as good as Tendulkar.
It's not possible to give a current-average for Kallis as he's playing in a Test at the time in question. However, if he doesn't bat again it'll be 72.49 between 7 September 2001 and the end of the Test going on at the time of this post. This, beyond question, is superlative, but coinciding exactly as it did (like several other players) with the deterioration in bowling quality, must be taken with a pinch of salt.
At the time that offered a much greater challenge for batsmen, Tendulkar outperformed Kallis considerably, though both were fine players. Since Tendulkar ceased to be the player he once was, Kallis has been able to considerably outperform him.