Do you want a heavy 120mm fan weighing on an adapater bolted to your case?
I'd go with the 80mm or maybe a 92mm if you can find a good one.
It is not really heavy, is it? And I do not think the case would bend because of the weight of one fan. No, really, I do not think that would be a problem.
I have currently a Papst NGL doing a decent job (80 mm). I am hesitating between either cutting the case and buying a 90mm Papst or not cutting the case and buying a 120 mm + adapter. I would rather not cut the case on the premise of being lazy. OTOH, 120mm Papst costs 25 euro here - the cost not to be discounted...