I'll take the MMA guy over the boxer any day of the week.
Boxers only know one thing: punch the guy opposite you. MMAs know a variety of disciplines: boxing, wrestling (real wrestling, not WWE), all the various martial arts (mostly muay thai boxing and jiu-jitsu).
Boxing also allows guys with probable concussions to take a standing 8 count. Any sign of an MMA fighter being unable to defend himself, the ref stops the match.