Keypads are nice because they will not disappear into a pocket or drawer somewhere and they are always available, even to users who don't have passwords.

Phones/pads are nice because they follow you around and probably display more meta data than keypads. Phones/pads are more likely to be updated to follow current trends.

Keypads are not very good at accommodating variations in vision or user height, but this may not be much of an issue for simple pads.