Your daughter has said how much she loves the dining room set your great-uncle built for your mother's wedding. So has your daughter-in-law. What to do?
Simple answer? It goes to your daughter.
Family decisions aren't always so simple, but the basic recommendation is to keep it along family lines. Even if your daughter-in-law wants it more, your daughter has been in the family a lot longer.
If it's your son who has expressed interest in it, it's not so easy. In that case, you can either take a guess at who wants it more; designate the dining set to your son and another heirloom of similar value to your daughter; or, better yet, ask them. You may find that one of them was just being polite.
Which brings us to the final major decision: When do you want them to find out what they've won?