I've read through this post on the differences between müssen and sollen: Difference between “sollen” and “müssen”
I was trying to decide how to say "you should have told him" (e.g. that jumping off a cliff was dangerous) but I'm confused about the difference between the following two sentences.
I had thought the translation would be:
Das hättest du ihm sagen sollen.
however the answer (in the exercise book) was:
Das hättest du ihm sagen müssen.
What is the difference between the two sentences?