This is common knowledge in the DB community, but out of curiosity I looked at the numbers:
Sawbonez: Medpack has 8 sec CD heals one ally to max. That equals to a ~115 heal on average.
Phoenix: With an 8 sec CD healing pulse you get 60-70 healing (could not confirm the initial healing value of the ability) on a single person.
This means a Medpack has the same heal/CD value as the healing pulse if you pick it up with ~60% health.
What does this all mean apart from this incredibly specific situation?
Phoenix is far superior in mitigating chip damage divided on the full team, but if the enemies are familiar with the term 'focused fire' and the incoming damage lands mostly on one person a good ol' medpack is the way to go.
There are other pros and cons for each two, of course: Sawbonez has 3 charges and overall requires less active commitment to healing, whereas Phoenix can heal mid combat, and has self revive, but the position in the meta of both medics can be explained with just the first observation.
TL:DR: Phoenix is the king of pubs with the instant AoE (Aura being the queen of course), but when people begin to focus their fire the full heals of Sawbonez are needed.