ID 4. Two similar cactus, I suspect they are the same species

Please help to solve the identification.
Two similar cactus, I suspect they are the same species.

Cactus 1
reddish stem
round white areolas
radial spines: 7, whitish with dark tip
central spine: 1, black, long

Second cactus has green stem
round white areolas
7 radial spines and one central, all smaller than those of cactus 1.
They are the same size and color, whitish with a dark tip

I think it's M voburnensis

Thanks

I don't think they are the same. I don't even think the second one is a Mammillaria at all.

OK, thank you jfabiao.
I wiil follow with the investigation.

I'd agree with jfabiao, and suggest that the second might be a young Eriosyce (Neoporteria maybe). To be a Mamm it has to have axils at the base of the tubercles, and I cant see any. The first is right, I think.

Thanks Chris