Who do you think is the better athlete: a professional mixed martial artist, or a professional boxer?

Your question doesn't make sense. The better athlete is the better athlete, whether he's training boxing or MMA.

Boxers have traditionally been among the best-conditioned athletes, if that's what you mean. An MMAer trains much like a boxer, so their conditioning will be roughly equal. Boxing matches tend to last longer than MMA bouts, so a boxer might need more cardio on average, but there will not be a lot of difference.

Because Boxing and MMA uses different parts of the body they would have a different level of fitness, an MMA fighter i would say has a better overall level of athletic ability while a boxer would excell at some things but lack in other areas simply because they concentrate on a certain area more

At the end of the day both train their asses off and just because some get paid more than the others is no indication at how hard they train

It comes down to how hard the individual is training, how his conditioning is, and how naturally gifted the individual is. For overall body conditioning i would go with MMA, but obviously some of the heavyweights in MMA aren't the best atheletes.

The mixed martial artist is the most complete athlete. Boxers are very fit, but it is a one dimensional sport.

Just watch MMA fights. There are fighters that are primarily boxers with limited ground skills, and most barely kick. You will see that almost every time the well rounded martial artist prevails in the conflict. However, I said almost because all the training a martial artist uses over their entire bodies is focused into a boxer's hands. There is always a punchers chance to win the fight with one well place shot.

a MMA fighter is a better athlete because their training are much more grueling, demanding and diversified than a boxer's. they have to train for so many various situations in their sport that it is almost impossible to say that a boxer is a better athlete.

I think the martial artist. They have to learn more kicks and punches and have more discipline. Boxers just punch people.

I think it depends. How long have you been training in each sport? There can be great athletes in each sport if you ask me.

Mixed martial arts competition requires stronger muscles throughout the torso so their vitals are protected. If a normal person were to take a shin kick from say, Bas Rutten, right to the kidney there would definitly be internal bleeding and extensive damage. The core muscles must be more than strong so that the fighter's heart, lungs, etc are protected from 1000's of pounds PSI on impact of kicks, knees, elbows, etc.

Depends, look at Tim Sylvia and Floyd Mayweather. Floyd is more of an athlete even though he's just a boxer.