The EVO (and the 840 that came before it) use TLC memory, whereas the "pro" uses MLC. This gets into voltage states and basically means that the MLC memory can handle more read/write cycles and has greater fault tolerances than the TLC memory.Weird that the evo is cheaper then the pro.
If you're into photography then this might seem a bit strange. Our CF memory cards are usually divided into SLC and MLC cards, where SLC is the more robust of the two (again due to voltage states). I guess SLC would be prohibitively expensive for use in a SSD.