This is purely speculation. My 12" MacBook has lived a REALLY tough life, and it's running perfectly.Get a laptop with a fan so the heat in the case is removed. The laptop will live a longer life with a fan.
Judging longevity based on heat is a massive oversimplification. Ask a PC guy about most Apple products operating temps and fan curves, they'd spit out their coffees and exclaim, "It'll die in a year!" Server hardware doesn't run cool, and that's with fans that equate to mini jet-engines. But that gear runs for years too.
I would consider the laptop with the fan if the one without the fan shows thermal throttling issues. If throttling isn't an issue, I'm taking the fanless version every time.
In fact, the fan in the 13" MBP is the one thing keeping me from pulling the trigger on it, and not the Air right now.