They're both very clever hard-working artists/businesswomen running their careers at different times in terms of popularity peak, context, media, etc. To fully grasp if Taylor surpasses M in any way, we'd rather wait a couple more decades, in fact. This doesn't mean her actual success isn't real at all. The figures are astounding. The money she makes and the influence she has within the industry are already huge. The cultural impact has to have more longevity in order to become a staple.