A 4-4 vs. a 6-1

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

Can you make six diamonds here if you get the finesse right? Nope. You'll hafta lose a spade and a heart. Can you make six spades by ruffing a diamond and dropping the queen on the third round? Nope. You can sluff four hearts from dummy on that line, but it can't do you any good if you have no trump left. Can you make six spades if you take the diamond hook? Yep. You sluff four hearts in dummy and avoid the heart finesse, ruffing a heart on the second round after unblocking.
The balanced over the unbalanced! The major over the minor! You have two principles here that direct you to choose spades. How many declarers would take the finesse, I leave up to you, but at least the spade slam is makable on a not-too-esoteric play of a finesse against a queen. And since you should be leaning toward a major when you have a real choice, the hand does call for a spade contract.
Here North heard his partner's rebid of a spade (after a diamond opening, a heart by North) and so has to know there's at least a 4-4 fit in that suit and so should correct the final contract to spades. If there are any holes in the spade suit, they'll be there just as much in a diamond contract as in a spade. Yes, yes, you can postulate circumstances where that doesn't hold. Let's say hearts are solid with A K Q on down. Yes, on a bad spade break, the diamond bidders might come out smelling like a rose, sluffing spades in the closed hand on the hearts. But this hand as you can see has neither solid hearts nor a bad spade break, and you'd do well to stick with likelihood, which is that with at least an 8-card fit, you're going to be stronger with the balanced suit as trump than the unbalanced, if there is a difference.
This is the second hand in recent weeks where the balanced major works better than an unbalanced minor and so the second time I have pointed out that pairs have two reasons for going with the major suit. The logical question now is, what if we have an unbalanced major and a balanced minor, going for slam, where we don't get that one-trick cushion we have going for game? Which principle should we pay heed to? I would go with the major over the minor, as just discussed in the the last example. You don't know that the balanced suit as trump would bring an extra trick (I have always pointed out that this is the likeliest case when there is a difference, not that there is a certainty) nor is it always easy for the one making the final decision to know that one suit is balanced, the other unbalanced. Sure I might miss a cold slam in a minor because I'm in an unbalanced major. But at least I'd get the above type hand right, where two principles steer you to fruition and would not be terribly ashamed to be missing a minor-suit slam here and there.
The hand just doesn't count out right without taking the diamond hook, especially on a club opening lead, and then club follow-up when you knock out the A of spades. Now you've been tapped once on the trump suit in the closed hand. Do you wanna cut down dummy's trump with a ruff also? I would hardly think that'd be very appealing. So if you're going to count out from the closed hand, I would suggest that anything other than the finesse is going to give you difficulty, putting you to the need to take the heart finesse, which has no greater likelihood of working.
Nor does the hand count out well from dummy, not even if the K of hearts is onsides! Let's see: we get a club pump at trick 3 (after a club opening lead and knocking out the A of spades). We can cash two diamonds and sluff dummy's last club and take the heart hook. The J is covered. Whoopee! But wait a minute. Now we've gotta take care of the third round of hearts in dummy . . by ruffing in the closed hand! That hand now has fewer trump than East! Now how do we draw trump and get back to dummy to cash out the hand? No, it just doesn't count out. After a club opening lead, knock out the A of spades, ruff a club lead, you've only two trump in the closed hand. If you use one of them for the third round of hearts . . . Need I say more? I think that 10 of spades would come to haunt you . . . now that's with an onsides K of hearts and a 3-3 break!
Gimme a break. I think you've gotta go with the 4-4 fit and you've gotta take that diamond finesse, and if it's off, so you're down, no? The diamond finesse, at least, gives you a fighting chance. I've pointed out that even an onsides K of hearts doesn't give you all you need.