ATX is better for standard size cases and parts, for gaming, high end use, as well as upgradability. Micro ATX is better when SLI/Crossfire are not being considered, when space is limited, when upgradability is not necessarily a priority etc.
Micro ATX is also better for some low budget machines due to the cost.
Are you looking at building a machine and trying to decide for yourself which to get?
micro-atx board are used in all those small form factor computers (theres even mini-atx which is even smaller) but i'd say they're used mainly for htpc type computers or for people who want a tiny case.
ATX is good because there are extra slots that you not got with Micro ATX however its not true at all Micro ATX do have all feature that ATX have but only on few boards
Size of both are boards are different
But people like ATX because they thinks its good than Micro ATX but its not true it just upon manufacture who to design it
No need to worry just choose what you need inside of it