You might start with the 2010 Gartner report on various BI tools:
It has sections with strengths and cautions for both QlikTech and Tableau, among others. I believe some of the cautions for QlikView have since been addressed. But then, I'm sure Tableau has been moving forward as well. I believe they had a major release very recently.
I can't really compare them personally, or at least not in great detail. I've looked into Tableau, and even downloaded it at one point. Unfortunately, I then got VERY busy at work, and by the time I had time to try it out, my trial period was over. But I've watched a few of their demos and the like, and have remained on their marketing mailing list. There are things about it that appear to be handled better than QlikView. And there were things about it that appear to be handled worse than QlikView. That's to be expected, I suppose. They seem to me like similar products competing for a similar market.