Freedom to Switch Dominant Suit

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

EastSouthWestNorth
Pass 2 Pass 2
Pass 3 Pass 3
Pass 4 Pass 4
Pass 4 NT Pass 5
Pass 5 NT Pass 7
All pass
The people in no trump fared far better than those in diamonds here. Indeed, 7 no sucked up so many IMP's that no other bid brought a positive IMP score (no one bid 7 hearts, which is also makable). Of course you get exceptionally bad luck in the diamond distribution and are fortunate in hearts that the J doesn't sit with the long holding. How can one know he's going to get good luck in the latter and bad in the former? Obviously you can't. But if you're in no trump you can do well for yourself if either breaks favorably. When one doesn't pan out, you go to the other, while in a trump contract, you cannot do that if you've chosen the wrong suit, as several did here.
In an earlier discussion of no trump vs. trump slams, this ability to abandon a badly breaking suit to test another was given as one of the advantages of no trump. Others were that you avoid a quick ruff, you can take your winners in any order you please (which can be important if opening lead wipes out an important entry in one direction or the other) and brings the higher raw score which is especially important in matchpoint scoring, and to a lesser extent, vis-à-vis the minors. (In IMP's, the difference between a major and no trump is negligible.)
How do you know when to move toward a trump slam and when a no trump? With the usual caveat that no one's going to get 'em all right and a further caveat that there are no rules cast in stone, I will offer the following: I would go for a trump slam on any balanced fit, defined as at least 4 cards in each hand. Consider these hard figures: if you have 3-card support for your partner, there is no chance of a trump left in dummy when all trump are out if he has a four-card suit and no chance if he has a five-card suit, and only a 40% chance if he has a 6-card suit! But if you have four-card support, there's a 68% chance of a trump being left in dummy after drawing trump if your partner has a four-card suit, a 90% chance if he has a five-card suit and a hundred percent chance if he has a 6-card suit.
Comparing zero, zero and 40 with 68, 90 and 100 should give you some idea of how valuable four-card support of partner's suit can be for your side. And my observation is that on a balanced fit, declarer will far more often than not have a ruff coming (easily) for a safer contract, maybe even a one-trick-higher contract. And secondly, a look at the obvious: If you're not quite certain that your side has every suit stopped by high cards, then the trump contract has to be safer. Did your partner cue bid another suit after a trump fit was found? Are you sure it's an ace he's showing and not a void? Actually, I'd go a tad further and say you'll probably be safer in the trump contract with a single stopper in a side suit, say a stiff ace. When that single stopper is knocked out on opening lead, you're often terribly vulnerable on finesses (and declarers often then fear them).
And all the rest for no trump? I would say so. With an unbalanced fit, you're not getting any ruffs in the short hand, or aren't so likely to. The no trump denomination allows you to roam from one suit to another, as above, no ruffing potential to surprise you, etc., etc. Well, look for yourself. OKBridge offers a wonderful opportunity to surf for slam hands. Look to see the dangers of trump denominations as well as the danger of no trump denominations on other hands. Ultimately, you'll have to be self-educated, for no mentor or teacher worth his salt would exhort you to take his pronouncements as the final word, not to be weighed against your own observations.
The above hand is almost too obvious a case of no trump being more attractive. South has every suit but his partner's twice-bid hearts doubly stopped in high cards and his partner has shown no particular liking for diamonds as trump but mere preference. South has to figure that . . . yikes, I just saw that it was North who made the leap to 7 diamonds! On a doubleton opposite a once-bid diamond suit! Since North's presuming (rightly, to be sure) that his partner has the controls for a seven bid, running 13 tricks without let-up, how can a doubleton Q make diamonds more attractive than 7 no? It just looks to me that with such exiguous support in diamonds, no trump would have to fare better, in points if nowhere else. But as discussed above, there's a far more important advantage here in no trump than just the scores of no trump vs. a minor. This suit doesn't split right? Okay, then I'll try this other suit.