Nano S vs Mini C?

[Tilltech]

They are practically the same size except that Nano S has included 140mm front fan, while Mini C has a 120mm front fan.
The other difference is that Nano S has shorter, 162mm CPU cooler clearance.

Still, Nano S is cheaper, and no need to buy two 140mm fans, instead just one extra.

Which one would you pick for a high-end gaming PC?

I guess, it's all up to CPU cooler size then?

[CA_Steve]

162mm vs 168mm on CPU cooler not a huge difference...especially as there's only a few 165mm+ coolers I'd recommend. More important is the PSU space. Nano S limits to 160mm. Mini C has a big area (max 228mm)...but...you need to stuff all the cables in there (so, modular is the way to go) and it butts up against a drive bay..unless you remove the bay. So, tradeoffs.

[Tilltech]

It turns out that ITX MBOs are very expensive and have less slots and features, despite being more expensive.
I don't think it would be even possible to have a sound card(like ASUS STX II) along with high-end GPU on it.

So, Mini C would be the way to go.