The main difference between these pasta roller and cutter attachments is that the KSMPRA (roundish housing) is a newer model with all metal parts, while the KPRA (squarish housing) pasta maker attachment contains a metal housing with some plastic parts. If you take a look at their product pages, you will see that the KSMPRA is marketed as a commercial unit, whereas the KPRA makes no such claim.
On the other hand, it is always important to read reviews before you buy and one customer who has owned both has pointed out that the older KPRA model is easier to clean because there is a higher tendency of dough getting caught in the KSMPRA pasta maker. A user on Reddit also points out that they contacted KitchenAid to clarify that the above is true about the metal and plastic parts.