I prefer NewEgg, as the Product Spec's are superior to just about every other site out there. I have seen MANY sites that list like 3 small aspects of the product's spec's, which to ME, is a pain, because I don't want the hassle of researching the product just to make sure that is what I want. Also, they ALWAYS have deals going on. Also, I get the regular shipping, and haven't ever waited over 3 days for parts. The compare feature is nice too...
I have only used ZZF a few times to look products up, but I don't like the way the site is layed out, and also, the prices (on the few components I was looking at) were more expensive than Newegg, even after shipping...
Plus, I had the misfortune to have to RMA a DOA PSU to Newegg, and it went smooth, VERRRRY smooth... It was only a matter of days by the time I got the replacement.