This topic has come up before, although I can't for the life of me find the post. In that topic, the general consensus was "it depends". On what exactly might affect that was the general course of the debate, but no definitive answer is available since we do not really know what percentage of men would make it through every year of service. For an individual entering the Auxiliaries, what would make a difference was really about whether they would end up on any sort of campaign. Up until the modern period, by far the biggest killer for soldiers was disease, and on campaign in tight quarters the risk for such would be increased more than the risk for death from combat.

The closest one could come to trying to figure this out would be to try to calculate how many men were enlisted each year in the Auxiliaries during the Pricipate, then try to extrapolate that by using some of the legionary diplomas that have been found. But this is at best educated guess work, and at worst woefully inadequate speculation.