A full ATX offers more flexibility than a Micro ATX or a Mini ATX.
usually any motherboard that has a form factor above mini or micro atx (atx, extended atx, super atx) will have atleast 4 if not more expansion slots available by default and normally an atx (and possibly a extended atx) motherboard should fit in a full tower case (a super atx motherboard will more than likely need a super tower)
There are no similar form factors to ATX if you're talking about a slimline. Slimline form factors are NLX and LPX. The form factors to ATX are BTX, Micro-ATX, and Flex-ATX. There is no correlation between slimline and ATX.
ATX is the form factor.
ATX
Advanced Technology Extended Switched Mode Power Supply
It's an ATX Full tower Chassis and all ATX form factor power supplies should work with it.
ATX is the form factor which Motherboards, Computer Cases and Graphics Cards use to standardise sizing. An ATX form-factor Motherboard will fit an ATX Case, and an ATX Graphics Card will fit in the case as well.
The most popular form factors are: -Mini ITX -Micro ATX -ATX -E ATX
The full ATX motherboard size is 12 in long by x 9.6 in wide (305 mm x 244 mm). Since it came out in 1995, the ATX computer form factor has replaced the old AT form in virtually all computers. ATX means Advanced Technology Extended.
Some motherboard form factors (from smallest to largest in physical size) are: -Mini ITX -Micro ATX -ATX -E ATX
The three form factors for desktop system boards are: ATX, and BTX and NLX