The Safest Place

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

SouthWestNorthEast
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 North, for which reason this hand has been entered in that category (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 suit for trump (i.e., if there is no prior preference indicated -- 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.
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 grand slam level. North has no particularly good fit with his partner's black suits (though 3 small spades are deemed "adequate") and each red suit doubly stopped (i.e., North would have each doubly stopped as declarer in no trump). 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 West 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. Here, of course, the sequence would be the opposite. Declarer tests clubs and finds they're not splitting benignly and so switches to spades and makes his no trump contract.
But this South bid to grand slam. Well, that's not less reason to pick a suit for which North 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. Clubs not running? Well, you learn this soon enough to avoid losing your last stopper in clubs, for you're going to have to finesse the spade 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.