If you're doing manual machining, the backlash of the machine probably limits it to conventional machining.
With CNC, climb milling is often preferred.  The exceptions when it is better to conventional mill are:
When milling rough or hardened surfaces.
If your cut width is 3/4 of the diameter or more.  Climb milling produces negative cutting geometry in these cases.
When the finish pass is tool deflection-challenged, prefer conventional milling.
Conventional milling is often preferred when Micromachining.