What I've seen in the biotech/pharma world is that both masters and PhD biostatisticians are very employable. The masters people tend to be the worker bees who eventually rise to level of say manager. But those who rise to be department head or director level very often have PhDs. I don't see a saturated job market for either. In fact jobs for talented, experienced biostatisticians often go begging.
The PhD degree by itself gurantees nothing. The main advantage to having a PhD is that you won't ever be held back or passed over for the big promotion for not having one. So I say if you want a PhD and you can financially afford to stay in school for another 2-3 years, go for it. The opportunity costs to your career in the short run are moderate and potential pay-off in the long run is significant.
(Disclaimer: the above assessment is for Biostats. I am less familiar with the situation for straight Statitistics grads in a non-biotech labor market.)