Exactly as the name implies. One is for slot A, one is for Socket A form. Slot A's are OEM's (technically, you shouldn't be able to buy them outside of a whole system, but people sell them in the gray market).
They have timing issues with the KX133 chipset from VIA, and the KT133 is the chipset from VIA that supports SocketA chips. the AMD 760 and 760mp chipsets do/will support Socket A as well.
Basically, if you have an old Slot A AMD 750 chipset mobo, the Slot A one is just fine, performance wise, given the same chipset and everything (which won't happen, because no one will make a Slot A KT133 based motherboard, nor a Socket A AMD 750 chipset based motherboard), they'd perform the same.
Biggest differece should be in AMD's eyes: Costs. Socket A is cheaper to produce than slots.