What kind of preference is that? Four diamonds and 2 spades, with a not very prepossessing honor and you prefer spades? Whew! In addition to Preference, this hand could easily fit in Don't Sneer at the Minors, Good Fits and The Forcing Game (which here is disastrous). In fact, it seems such an outrageous sneer at that fine diamond suit for a moth-eaten spade suit, that I believe I'll put it there too.
In my introduction to Preference bids, I said that when the choice is between a major and a minor, rather than rushing lockstep into the longer holding, I would ask if we seem to have sufficient support for the major suit, in which case I'd go for the major. "Sufficient" is traditionally 8 cards for a trump suit. Two spades just won't bring that second suit up to sufficiency. And when you have such mahvellous support in diamonds! But that's a minor! Oh, yes, of course. But take a look at the above hand and ask whether you'd rather be in spades or diamonds. Okay?
Supporting a partner with two cards is fine when a partner has shown 6 cards in a major, and sometimes you'll do it even on 5, when there is no alternative. Lemme see: if I had a king doubleton in spades, 3 no count hearts, 4 clubs to the Q and 4 diamonds to the K, for 8 hcp's and the bidding went, starting with my partner, one spade, pass, one no, pass, pass, two clubs on my right, I would bid two spades, especially not-vul. I think we should be competitive at the 2 level and my partner should know that I'm bidding on an honor doubleton, for with three, I would raise rather than say a no trump. Okay, so there's no argument that two-card support shouldn't be considered. But with four-card support for the first-bid suit? When 5 diamonds is virtually a laydown? Oh, but North doesn't know that. Oh! That's right, isn't it. But North doesn't haff to know that. He has to show his preference where he has the greater length -- with, yes, the qualification given above, which doesn't kick in here.
Let me cite some simple statistics On three-card support vs. four-card And that's with three! Because a major suit game is worth more than a minor suit game, I'd support with 3 cards if I knew my partner to have 5. But at the slam level, I would be sorely tempted to name the suit with four-card support.
Here, of course, there's not even three-card support, and it's just amazing that these people have been so conditioned to sneering at minors that they wouldn't think a two- card difference is sufficient reason to show preference for the minor.
Now, as for the play. At the top of the list (i.e., with the worst score) was a declarer down five. Down five on a freely chosen contract! The next went down three. But lemme look at the one who went down five, since the defense was more aggressive and declarer lost control: Ruff the opening lead, lead a spade to the 8 won by the 10, ruff another heart lead. Do you know how many trump declarer has at the end of 3 tricks? Two. And how many do the defenders have? Two each. And declarer has 8 tricks to go for his contract! Not to mention the A and J of spades!
Declarer cashed a diamond, and on a diamond continuation, East ruffed, led the J of clubs to the A, the 10 of clubs now, around to declarer's 9 of spades. (He now has only the K of spades in the closed hand.) A diamond was now ruffed by West, and another club led, declarer ruffing with the K, his last trump in that hand. Another diamond was ruffed by W, a heart went to East's K, and now the A of spades was cashed, taking the Q of spades, and the 10 of hearts took the last trick. Down 5 in a 10-trick contract means they took just five tricks. The defense took three more tricks on their 6-card spade holding than the bidders did on their 7-card freely chosen spade contract. Declarer's winners were one diamond and ruffing four times in the closed hand.
As for the preference shown, which is the main topic here, the first North above has little excuse for his choice. He's got two more trump for the first bid suit and chooses a second suit where he has only two-card support? The second North player clearly has a lot more justification for his choice (though I wouldn't say the same of South's choice). Indeed, I think I might well prefer 4 spades there myself on these grounds: My partner has gone to the four level all by himself, and I have a little stuff for him, while having promised nothing. Since we're at the four level already, which isn't game in diamonds (which I would pass if it were), and I do have an honor for my partner, I think we might as well slide over to a game bid. That extra trick required for game in diamonds, my partner's second suit, might prove just one level too high. Of course, I wouldn't expect such a hand as South is showing. To start with an anemic 5-card suit when you have a solid 7-card suit doesn't strike me as the wisest of decisions. I would indeed have looked for a strong spade suit and perhaps some holes in my partners second suit.
Or let me put it this way: If you reverse the suits, making his spade suit look like the diamond (substituting the J for the Q, which South could not have) and the diamonds look like the spades (also making allowance for duplicate 9's), which suit would you rather be in at game level? I don't think there would be much dispute about that, which is why my partner shouldn't start with a spotty five-card major over a solid 7 card minor, no?
Before leaving, perhaps I'd best say that this isn't chisled in stone. If both suits, a rather skimpy 5-2 major and a healthy 6-4 minor, are solid from the top, the major will almost certainly do you better. But if either suit is spotty and needs development, particularly if you need to lead it twice to establish it for running, especially when your control in a third suit is by ruffing, not by a high card, then I would say the safe and comfortable 10-card minor will almost certainly do you better.