ATX improves upon AT by making adding and removing components easier, providing greater support for I/O devices and processor technology, and lowering cost.
There are not really any advantages with a ATX over a Micro ATX just Micro ATX is smaller and ATX is larger
adding and removing components easier, better support for I/O devises and processor technology, lowering costs.
Baby ATx and ATx
A full ATX offers more flexibility than a Micro ATX or a Mini ATX.
Full AT, Baby AT, and ATX
What is Data Communication Equipment (DCE)
A baby AT will not fit into the same case as an XT on a motherboard
The four most popular motherboards types on the market today include ATX, Micro ATX, Baby AT, and proprietary boards. Proprietary motherboards are created by large PC manufacturers such as Dell, IBM, and Compaq, and are sold as part of their PCs.
There are more than 10 types. ATX, Enhanced ATX, MiniATX, MicroATX, FlexATA, BTX, MicroBTX, NanoBTX, PicoBTX, NLX, AT, and Baby AT
ATX
Size is the advantage. MicroATX is usually much smaller than normal ATX and usually has fewer expansion slots like PCI (+variants), fewer DIMM sockets (usually 1 maybe 2) they are often built around budget or low cost CHIPSETS. Much of this depends on the manufacturer. Some motherboards come with the CPU soldered on while others use a socket. Often mATX boards cost the same as ATX and if space is not an issue in your chasis, then go with a Standard ATX. Standard ATX provides a better upgrade path over µATX. ATX is just a way of describing the type of motherboard.
flex ATX above that is the micro ATX