I mean the second one. Also, if I said the first one, I would mean the second one. They're the same by the definitions I use. The second one is more clear.

If it is impossible to have off-diagonal results, that is a much stronger argument for cooperating than having it be improbable, even if the probability of an on-diagonal result is 99.99%; as long as the possibility exists, one should take it into consideration.

If the probability of an on-diagonal result is sufficiently high, and the benefit of an off-diagonal one is sufficiently low, that is all that's necessary for it to be worth while to cooperate.