Atx or M-atx for build

I'm building my first custom pc and have all the parts I want ready to be bought. The only thing holding me back is what size mother board I should get. I want to use the Fractal design arc mini because of its convenient compact size. Its a strictly matx case, but the company also has the arc midi, a full atx in mid tower case. Im using this computer for mid gaming and graphic design. I just wanted to know, should I get matx or atx. I understand the matx has less PCI slots, but do I really need like 4 graphics cards? The only reason I wanted matx was because of the case sizes.

So what do you think?

Thanks