For maximum tool life, cut full depth and full speed. The only reason to go slower is a lack of machine rigidity, power, or cut quality.
If tool life is your main goal, cut as deep and as fast as you can.Is one generally better, or are there situations where it's best to choose one over the other and vise versa?
But as I mentioned, there are other factors that come into play. Sometimes you need to cut shallower to achieve a better cut quality. You can do both; make several shallow roughing passes followed by a full depth finish pass. The tip will still wear out faster this way, but you can get a cleaner cut over most of the edge this way.