Alrighty, the skinny on everything - ITX vs mATX.
To be simple about it - it boils down to expandability. ITX is very limited in regards to options due to smaller real estate. It's pretty tiny, and space is at a premium. mATX has much more space (96 sq inches) vs ITX (44 sq inches, or less than half) and between the rear IO panel, the CPU block, and the RAM block, there isn't much room left. Meanwhile, with twice the available space, you have a lot more room. LOTS.
In general, things that you can have with a mATX that you can't have with ITX:
1) more RAM. Most ITX boards come with only two slots. Not a problem these days as you can get larger DIMMs to make up for it, and most computers are just fine with 16GB (2x8GB). But, should you want to go bigger down the road, you're limited.
2) more storage. Most ITX boards only have 2-4 SATA ports. But, with a limited build if you wind up with only 2x SATA, that leaves you with a boot drive, maybe an optical drive or a large bulk storage drive. Meanwhile, most mATX boards have the full complement of 6x SATA ports. Again, you have more options and expandability down the road.
3) PCIe/PCI expansion. ITX has exactly one PCIe x16 slot. Allows for one graphics card. mATX meanwhile has provisions for 4 slots (mix of x16/x8/x1 or PCI legacy). Again, it gives you options - if you decide to add a secondary graphics card, or more storage, or video capture, or anything - then you have a choice. While these days SLI is being depreciated by nVidia (with good reason) - with an ITX board you only have one slot for one video card. Period. (just as a frame of reference, I have a secondary nic, twin 960s, raid card, and I still have three more slots on my full ATX board).
4) M.2 drive support. Many newer boards are coming with M.2 slots for the small SSD drives. Some ITX do. Makes a build a little easier.
5) WIFI. Many mITX boards come with built in WIFI via built in to the board, or on a mini-PCIe slot. Because ITX has limited space they provide for this option, most mATX boards do not, and getting WIFI is either a USB dongle or use one of your expansion slots. Regardless, it would cost extra which is offset by the slightly higher cost of ITX boards making it a wash.
6) overclocking options. Usually because of the limited space, there's less room for a robust power delivery system on an ITX board. In addition, because they're put into (usually much) smaller cases, they don't have the volume for handling excess heat. On an mATX build, they're put into slightly bigger cases, more room for airflow, and more board space for better power delivery, so they have more OC headroom.
So - all in all, mATX gives you more options to upgrade in the future, but if you plan your build appropriately there's no reason why an ITX build wouldn't be just as good. If even think you might want to expand in the future (build in additional storage, perhaps go with a secondary video card, etc) then go with mATX. If this is a dedicated build and you don't plan upgrades, then build a tiny ITX build and get a decent little card (GTX 1060 or GTX 1070, or a RX 460/470).
Let us know how it goes.