I believe you're talking about potamotrygon hystrix.
They are one of the smallest rays, relatively cheap, and pretty easy to take care of which makes them a great starter ray. They except a wide variety of foods and can be pellet trained pretty easily.
As for how they compare to motoros, they stay allot smaller but are just as hardy and just as excepting of many new foods. They are allot harder to find and a little bit (most of the time) more expensive than motoros.
Its not easy to find true hystrix though. If you saw it at an lfs, I'm 90% sure, without even seeing a picture, that its not a true hystrix.
If you're interested in some, I believe spotfin has a few pups left and i remember there was a guy selling an adult pair not long ago.