Simplest answer depends on what your goals are...
A smaller, compact build would like the mATX board. A large build with a tempered glass case that showcases a vertical mounted GPU amidst the glaring and glitter of RGB fans and coolers might look better with an ATX board to not leave the case looking so empty, for instance.
300 vs 400 chipset isn't really an issue as they'll most likely perform very similarly for a 1700 and 2666 ram. Even if you want to slip in a 3800x and 3600 memory in the future they're both very capable. Getting into the 3900/3950 world may be questionable for either and nobody knows for 4000 series.
The remaining depends on what features one might have that tickles your fancy, if any. look at USB ports, pcie slots (ATX has more, but they aren't as useable as you might think), SATA ports and audio.