Whats the difference between 600 and 800 fill down? Is it just the weight? Basically if two bags are rated to a certain degree but one is 800 and one is 600, is the 800 fill bag likely to be more accurate or warmer?

I'm looking at getting a zero degree bag but I don't know if I have the money to spend on an 800 fill bag and I'm wondering if I can still get a good bag that is 600 fill and accurate.

MTS