Why switch from Atahualpa to Montezuma

Hi,
the title pretty much says it all: Why would i want to swtich from Atahualpa to Montezuma? Should I, anyway? What would be my advantages? More performance (i.e. less bloating, faster response times, less css, less anything)? Better mobile compatibility? Better compatibility in every area (I imagine the HTML5 thing might be a problem for people with older machines - is my assumuption wrong?)?

I am interested in everything that improves the site experience for my readers (and does not deteriorate it for anyone) so I am sincerely interested in this. Is Montezuma just better than Atahualpa when it comes to end-user experience?