That's because it depends on the design. On the lifetime point, for example: if the machine tends to fail based on time spent running (solder creep, perhaps), leaving it running more often will reduce the life, but if the machine tends to fail based on power cycling (low-cycle fatigue, perhaps), turning it on and off more often will reduce the life.

Given that I've dropped my MacBook from a height of four feet onto a concrete slab, I figure the difference is roundoff error as far as I am concerned.