Give a reason if you can.
It's a good question and I know I think truth is far more important but this question comes up: if your son wasn't really your own, would you want to know?
I like to think I would but being in the situation would elicit a different response, I imagine. It's tough to know for sure with this duality of man.
@OP
It definitely depends on the situation. You'd have to use your head and make the relevant choice.
Truth, definitely. If you really care about people, you'll be truthful to them. You can't just pat people in the head and tell them everything is alright, they won't develop into decent human beings if you do that.
I'm going to be a bit more precise about this: Honesty or acceptance of constructive criticism when relevant (something I consider to be honesty with oneself) vs regard for feelings (yours or other's).
Not implying they are always mutually exclusive: in a case in which they are mutually exclusive which would you choose?
Truth, and Compassion are not mutually exclusive.
Both are very necessary for meaningful human interaction.