Here's my informed opinion. I do design work in a renovation company - one of the things we do is kitchens - and many different types of countertops.
I like quartz better than I like granite - in fact I will be getting it for my own home.
Quartz can look like granite, however it is far more uniform so that seams and the like are less visible.
It has no fissures or pores - it's actually certified by the National Sanitation Foundation for food areas. Granite is not. (Silestone in particular has MicroBan in it's mix - so it is even more resistant to bacteria)
It never needs to be sealed - granite does.
It is as scratch resistant, or even more so, than granite.
Quartz has a warranty when installed by a certified installer; granite does not have a warranty.
There are many advantages of quartz over granite. That's why we are doing SO much of it lately - along with Corian (and the like).
Hope this helps!