Generally heavily armored gladiators, like the murmillones would sustain fewer injuries than lightly armored gladiators like retiarii. However, it's important to note that the designers of the games would generally pair matches in classic ways. For example: The quick, light retiarius vs the slower, stronger, heavily armored murmillo. This classic agility vs strength battle doesn't necessarily lend itself to a highly predictable outcome. The fighting styles are different and therefore the outcome would be highly dependent on individual skill.

Later, the secutor was developed as a variation on the murmillo with armor changes to counter the advantages of the retiarius, such as a grated face covering to protect from the trident and a smooth helmet to resist being caught in the net. It was not uncommon for the retiarius to be faced by two secutors at once. These variations seem to suggest that popular opinion was against the retiarus and they would be expected and perhaps preferred to lose.

As a general rule of thumb, I would say the highly armored and equipped gladiators would have a higher survival rate. Gladiators that survived to fame were of such sort, like Spartacus, a Thracian. The emperor Commodus, who liked to participate in the games, chose to fight as a secutor. While it is unlikely that he would be allowed to lose, his choice of fighting style is telling.

Additionally, it's important to note that the gladiator wouldn't have much of a choice in the matter. Some, like the Tracians, simply fought with their native weapons. Mostly, the gladiators would be allocated based on body type, with stockier, stronger individuals becoming sectors/murmillones and more lithe ones as retiarii.