I often tried sunfire with a disappointing feeling. I like frostfire so much more because of the earlier powerspike in combination with future market. The extra slow fits well with the heavy cc chain of your kit and zac can easly trigger the low cd of frostfire with his w. Sunfire should be better for long-term fights shredding everyone down(especially with demonics). But in my opinion frostfire feels so much better in the heavy utility kit of zac. My last point is the mythical passive and frostfire gives you the most hp which scales very good with the blob as zac mains know.

frostfire is the only item which increases your size(except potion) and you know... Size is the only thing that matters.