An ATX motherboard is a few cm longer than a mATX one; practically speaking, this allows it to have more expansion slots (e.g. PCI or PCI Express) and potentially more headers for things like USB and SATA, than a mATX motherboard would.
mATX motherboards are generally cheaper than equivalent ATX models.
A mATX motherboard will fit an ATX case, but an ATX motherboard can not fit into a mATX case.
AND YES MAXIMUS V GENE IS A GOOD CHOICE. ITS A mATX MOBO BY THE WAY