I never really got to see much of him live, as he unfortunately only played fleetingly for Pakistan, but what I have been told by people whose knowledge of the game I respect, he was much quicker than Akhtar, and probably had the best physique of any fast bowler around at the time. What eff'd him up, was a crap captain over-using him on a lifeless pitch, which caused him to bend his back much more than normal, in order to get his nominal pace. Also, in every feature on him, some of the world's and Pakistan's greatest like Wasim, Waqar, Lara, Aqib Javed have attested to his talent, and even Shoaib himself, have stated that Zahid was comfortably faster. Lara even said that one over which Zahid bowled to him in the 1996 World Series, was the fastest over he had ever faced by any bowler.
I've seen Akhtar for all of his career, trust me when I say this - Akhtar is a shit bowler, and a waste of a prodigious talent. He was a good prospect during 1997-1999, but he never progressed. He never learned to improve his line and length, and he was continually given chance after chance to do so. If you want a perfect proof of this, look at his performance against India in 2003. He was bowling at his fastest then, yet what he was serving up was long-hops which Sachin and co. simply lapped up.
Also, I wouldnt take much credence in his stats. He had a very good start to his international career, but he has regressed since. His Average in 99', for example, was in the high 18s, to low 19 range. I dont doubt that he was, and still is a box-office entertainer, but he could have been so much better. We should be talking of him with the likes of Lillee, Thompson, Holding, Roberts, Khan, Ambrose - but we cant simply because of his own selfish character. When people remember him they'll say "He bowled 100mph twice you know" and not to reminisce over any legendary performances.
I'd rate Brett Lee ahead of Akhtar, and I have done since 2001. Akhtar is a shocking waste of talent...