The Safest Place

A Q J 9 5
------
Q 2
A K 8 6 5 4
K 10 8 7 2
K J 10 8 5 7 6 4 2
8 7 6 5 3 10 4
2 J 10 9 7
6 4 3
A Q 9 3
A K J 9
Q 3

NorthEastSouthWest
1 Pass 2 Pass
3 Pass 3 NT Pass
6 Pass 6 Pass
7 Dbl All pass
What are they doing in 7 clubs? North-South had three reasons to be in spades over clubs and one to be in no trump. What are those three reasons? Well, first, foremost and most unambiguously is the preference expressed by South, for which reason this hand has been entered in that category (rotated 180 degrees, the only appearance besides this one). Secondly is the advisability of going for a major over a minor when both seem suitable for trump. Thirdly, you wanna prefer the balanced over the unbalanced trump. I have said under "Balanced over the Unbalanced" that the major suit over the minor will probably do you right five or ten times for every time the balanced over the unbalanced does. This, of course, is a very evanescent priority when it's number three. Still you might note that by chance, you can ruff out the third round of clubs in a spade contract (though you don't need to here). That is to say that after two rounds of trump, you can cash two rounds of clubs with impunity. You figure that if the suit is breaking 3-2, that second round is safe and you return to drawing the last trump. And if the suit is breaking 4-1, you're in trouble in grand slam unless the person with a singleton club has a doubleton spade, in which case you can ruff the third round of clubs, get back to North hand above, draw the last trump and claim.
And lastly is the advantage of no trump slams when everything is stopped by high cards -- and you know it, or in any event, have presumed it (as here) by bidding to the slam level. South has no particularly good fit with his partner's black suits (though 3 small spades are deemed "adequate") and each red suit doubly stopped with South as declarer (and in grand slam, South'd presume that the opening lead would be the only time the defense is on lead). The grand slam, though makable in spades or no trump, is a tad aggressive. Let's say you stop at 6 no. Now, spades don't have to split benignly. You're missing the K and 10 out of 5 cards, and East could easily have K 10 x x, to defeat any spade slam.
So in no trump, you switch to clubs to see if that suit's splitting benignly, which switching you can't do to advantage if you picked the wrong trump suit. But this North bid to grand slam. Well, that's not less reason to pick a suit for which South hasn't shown any liking and for which he certainly could have had less support on that bidding. You'd be far happier in no trump. First you test clubs. Whoops! Not running. But you learn this soon enough to avoid losing your last stopper in clubs, for you're going to have to take the spade finesse into the hand with the long clubs. But the finesse wins, and you romp home with 5 spade winners, four diamonds, 3 clubs and a heart. Not bad for a couple of amateurs.