zektro :
My idea was that I was thinking to build a custom sub-compact light/mid range gaming RIG from scratch, and yet I was wondering if a B350 mATX MoBo is worth it like the MSI B350 Mortar Arctic. Or, should I buy the ASUS STRIX ROG X470-I Gaming which I suppose that can be a better choice?!
Look at it this way: the MSI B350m Mortar needs a larger mATX case and thus won't make nearly as compact a system as you can build with the ASUS X470-I for which there are a lot of really tiny (and nice) mITX cases available.
The ASUS board may be more expensive but then you have to buy a WiFi card for the MSI which will boost total cost for it.
The ASUS may have better VRM section and so may overclock better but that's rarely exploitable in sub-compact systems' tiny cases because of cooling limitations so this is probably a wash.
But on the flip: If you don't really need/want WiFi and can compromise on just how compact your sub-compact system needs to be you can save a lot if you get the MSI board since not only the board is cheaper but those tiny little mITX cases for the ASUS are generally more expensive too.
That should give you enough to make a choice if being 'sub-compact' is your only criteria for success.
EDIT ADDED: Oh yeah, the b350 chip on the MSI board will work perfectly well with the new Ryzen2000 (Zen+) chips but it appears to me that it doesn't support that CPU's enhanced XFR-2 or Precision Boost Overdrive, which is a bummer. Maybe one of the AMD experts could comment on this, though, as I've not seen any comparisons of the new CPU's on the old chipsets vs. the new chipsets to see how much that compromises overall performance. But remember that when you overclock any Ryzen CPU it disables XFR and Precision Boost anyway so that's a wash if you plan to overclock. I just wanted to let you know the MSI board is very much still relevant in 2018.