I got a request to do a comparison between the two so here it is. :)
Honestly, though, I haven't used my Sigma F80 enough so this is more like my initial thought. I actually prefer my Everyday Minerals Kabuki Brush to apply my foundation because it gives a lighter, more natural finish. On special occasions I do like to use my Everyday Minerals Flat Top Brush to get a better coverage. I've owned both for almost three years and they still work well. They don't shed much either despite the fact that I wash them at least once a week.
From their appearance, you can tell that Sigma has a longer handle and shorter bristles than Everyday Minerals. Personally, I find the handle of Sigma F80 rather awkward (I wish it was slimmer) but I guess it's something I will eventually get used to. On the other hand, I like the short handles of my Everyday Minerals brushes because they're more comfortable in my hands and more compact, too.
As for the brush head, it's obvious to me that F80 has more bristles than Everyday Minerals Flat Top. Its bristles are definitely denser too, allowing me to get a precise and flawless application.
I didn't notice any difference with how the bristles feel on my skin until I used F80 on my face. It felt softer than my Everyday Minerals Flat Top but then again my EM brush has already gone through a lot of washes.
Since Sigma F80 has slightly shorter, more concentrated bristles, it transfers and blends product more effectively. With EM Flat Top, you will have to put more effort and time in order to get the same kind of finish.
Will either of the two make you look like you're wearing a mask? I think that significantly depends on what foundation you're using. My foundations generally offer medium to full coverage so yes, in my case, if I'm not patient enough to blend my foundation, I can end up looking cakey. This is why I prefer a dome-shaped kabuki brush for my daily foundation brush.
If you haven't purchased any of these, I'd recommend getting F80 simply because it does the job better. It also doesn't shed (probably a couple of bristles TOPS) or bleed when I wash it; definitely a good, effective brush for the price. The only thing that I don't like about Sigma F80 is that it's pretty hard to
get the foundation stain off. I've washed it thoroughly with brush
cleanser but take a look at the above picture... that's the best thing I could do with it. *sigh*