As for me, I think the Focus is a better deal. I note that Mazda is trying to dig themselves out of a hole of their own making (i.e. that butt-ugly / smiley face / uglified rear-end major styling screw-up)...
Having said the above, I would offer the following comments re the M3:
I contend that the M3 has more built-in sport than the Focus 'cause -
i) the TCS / ESC "tone-down" switch is actually on the dashboard. The Focus', only affecting the TCS -- and truly turning it off, is buried several layers deep in the interactive display [thumbd];
ii) the gearing on the SkyActiv 6 speed -- is a somewhat sportierr arrangement than that in the Focus 5 speed. The gear-spread between 3rd and 4th gear is a lot tighter, ratio-wise, than that of the MTX-75 in the Focus;
iii) electrically-driven hydraulic steering: it's clear that steering feel is utmost in Mazda designers' minds.
I am not sure what the comparative 0-60 times and Quarter times are... but it may very well be that the M3 is a sportier, more capable, quicker drive, on a twisty two-lane road, where the sport of overtaking slower cars can be liberally practised
I can't help but thinking that Mazda engineers tend to bake in more inherent "sport" into their cars, as another person aptly put it.