There are at least six good subspecies for Mammillaria rhodantha, all of which were individual species at some point, plus at least a handful of other species which have been completely submerged. That should give you some idea of how variable M. rhodantha is in the current nomenclature.

Subspecies pringlei and aureiceps tend to have quite strongly yellow spines, ssp rhodantha somewhat reddish, and the others more strongly red spines.

The one on the right looks like a fairly standard M. rhodantha. There's normally some wool at the crown but I don't see it.

The one on the left looks like one of the yellow-spined forms such as the very common M. pringlei. M. pringlei tends to have one, sometimes more, particularly long spine curling up towards the crown.

The one on the left could be a M. rhodantha ssp. pringlei

Dawn

Thanks for the info. I didn't know that M. Pringlei was subsumed into rhodantha. How do you keep up with the changes?

I don't see any wool at the crown of the reddish spined plant.

Might this also be M. rhodantha? It was not labelled.
{{gwi:462705}}

It has that "rhodantha" look :) There aren't many Mammillarias with five-plus straight central spines and small pale needle-like radials like this.