Posted 2016-March-01, 08:00
I answered "science" since, if you are a blaster, you use a weak NT and get to the NT game on 1NT-3NT.
But the reality is that 5D is a superior contract at IMPs - at any vulnerability state but especially when your side is vulnerable - to 3NT.
You have to pick up the diamonds for no losers to make 3NT or 5D. If you do, you score 460 or 660 for making 3NT and 400/600 for making 5D, losing 2 IMPs IF the other table plays 3NT and makes it. That is a big if because with 9 clubs outstanding, a club lead is the likely lead and they are going down. And if they go down, they are going down 1 or 2 tricks, depending on whether they pick up the diamonds (or if hearts split 3-3, but almost always down at least 2 when clubs split 6-2).
In 5D, the worst you are doing is down 1. So, when they go down, you are pushing or picking up 2 or 3 IMPs when you don't make it. So you have a slight advantage in diamonds.
But the big potential payoff comes when you do make it. If they play 3NT at the other table, they likely will be set and you will gain anywhere from 10 to 13 IMPs. And when they make it, you will lose 2 IMPs. Clearly, it is right to play 5D rather than 3NT at IMPs.
(The analysis is quite different at MPs.)