Sorry, just answered to the title.
I really don't know which option is better for the lifespan of the fans, if they stop working it will probably be a matter of bad luck and not that much about aging at any mode.
As 13thmonkey said, anything under 60ºC is perfectly fine (and probably even under 70ºC) but anyways I won't concern about idle temps and fans behavier at all. I don't know how good is that card in cooling terms but even before changing the thermal (if you are getting high temps under load) I would try to improve the airflow of the box, because it is rare to get a good thermal gain repasting the GPU, but the temperature in the box matters a lot.