In opposition to Eriks opinion an external watchdog was always better, since it was always enabled and can not be disabled.

All internal watchdogs, which I know, are very tired after reset and need always to be awake after a (hopefully) proper reset. Additional it can be disabled again by software.
So an internal watchdog was only good to reset on your own software bugs, but never on electrical effects (noise, spikes, inproper reset on short power breaks, ...).


Peter