They can both be used as a timer to trigger an action. The tween behavior has additional features, settings, and options for tweening values over time, because that is what it was made for. If all you need is a countdown timer, use the timer behavior, because that is what it was made for.
There is likely no measurable difference in performance between the two. If you do manage to measure a significant difference, use the better one, and let us know too, thanks.