Sobers, when bowling with a new or semi new ball would produce unplayable deliverious, prodigious in-swingers, away cutters from an awkward length, yorkers and sudden unavoidable bouncers.
Kallis almost never appears to be doing that. He relies heavily on the batsman making a mistake.
Having said that about their bowling. It must never be forgotten that Sobers will sit comfortably in any shortlist of five greatest batsmen of all time. Some may argue against it but he is not a misfit and many will agree he belongs. Amongst left handers, almost all those who saw him play wil put him in a shortlist of one.
Where does that leave Kallis the batsman - no where in the picture I believe - not in the same picture at least.
As a fielder, Kallis is wonderful at slip - so was Sobers. He was as brilliant but he was equally brilliant at gully, leg slip, short leg and anywhere in the outfield. He had an explosive and unerring throw. In whats coming close to half a century of watching Test cricket, one hasn't seen a more complete all round fielder. Again, Kallis doesn't stand comparison inspite of being one of the finest slip fielders of his own time.
Their bowling is where they have the least difference (gap in class) and the batting where they have the greatest but the differences are there.