I spent some time this afternoon looking at pictures of both species, and reading about them, and have concluded that both species have wavy ribs, and both species have a lot of them, thus the literal meaning of the Latin species names would apply to both plants.
That said, and given the extreme variability of the individual species (which Anderson calls "a taxonomic nightmare", adding "spine color and number variable" for crispatus), the two species are hard to resolve when they are young. Based on the reference by Anderson, these are the differences I would think are most useful. S. multicostatus has potentially more ribs (as many as 120, compared to 25-60) when mature. S. multicostatus has 3 central spines, whereas S. crispatus has 1-4 when mature (though young plants have fewer evident centrals). And of course the origin speaks to the species involved (multicostatus is from the northern states of Coahuila, Chihuahua, Durango; crispatus from Zacatecas in north-central Mexico south into Puebla and Oaxaca).
Based mainly on those criteria, I see no reason to move any of the 4 plants currently in the database for S. multicostatus, but I'm open to some convincing.