How is it better?
It's cheaper, yes, and more than enough for almost any system, but I don't really see any way to consider it better. Of course, both are excellent PSUs, and I'd trust either of them.
Don't get me wrong I have a corsair 1000w myself but if I had to do it again I would get the 850w without any hesitation. Here are my reasons.
1. One 12V rail VS. using two transformers with a single rail in each.
2. Better quality components inside the new HX850 VS. the HX1000.
3. The HX850 is 80 Plus Gold and I am not sure if the HX1000 is even 80 Plus Silver.
4. The most important of all reasons is the 7 year warranty Vs. 5 year.