Which way is it better to set your engine, rich or lean on top?

Rich on top should really protect your engine of not overheating.
But this is a little slow to get high rpm/speed. But sure to finish the race.

Lean on top might cause overheating when you change the rate of gunning
the gas. Also on longer runs you might keep on pushing the car that will
tend to overheating. However, this setup lean on top is faster for the
engine to reach high rpm/speed. Also I get better lap time in this setting.

What can you say on this? What would you prefer?