Careful. There are two completely different words--pronounced the same--"discreet" and "discrete." I will just discuss "discreet." "Discretion" is the usual noun form of "discreet." "I'll leave the door unlocked." "That isn't good discretion." "Discretion" can also mean "good judgement." "What flavors of ice cream should I buy for the party?" "You know the guests best, use your own discretion." I am actually surprised that "discreetness" is a word, and I double-checked a dictionary, to make sure. You're right. It is. It sounds clumsy and awkward to me. I like the word "discretion" much better. There's a famous saying--a quotation from Shakespeare--"discretion is the better part of valor." It means that it is better to be cautious and avoid risks, instead of being brave about taking them.

Discretion would be the most common. By the way, the word discreet is often confused with discrete. Discreet involves being careful and not overly public, while discrete means something that is separate and distinct.