Nice plants, Simon, and again a bit of a challenge from photos.
M. laui ssp laui has stiff spines, M. laui ssp dasyacantha has spines that are more flexible, and M. laui ssp subducta has yellow centrals, so we can rule this last one out.
Between the other two, ssp. dasyacantha is supposed to have smaller heads than ssp lauii.
So I guess its up to the finger test - from the look of the two plants the one you want to identify specifically might therefore be ssp dasyacantha, but it could also just be variation due to cultivation.
Do let us know what they feel like - if there is a difference, or even if there isn't.