Major over the Minor

Experienced player are very strongly major-suit oriented, which is to say, they will look eagerly toward a major suit fit as pretty much their first priority, and if there is any choice between a major and a minor for trump, they will go overwhelmingly for the major. So we have a system based on five-card majors (i.e., an opening bid of a major promises five cards) and the Stayman convention designed to smoke out a 4-4 major-suit fit, with no corresponding system or convention for the minors, not to mention individual proclivities, such as bidding a 4-card major opposite a takeout double even when holding 5 or even 6 cards in an unbid minor.
The reason for this is rather self-evident and inherent in the terms: you get more points with major suits, even more with fewer tricks won starting at the three vs. the four level. And this translates out most dramatically to game at one trick lower. Which is a big trick. When you move up the ladder from 10-trick hands (i.e., when either side can make 10 tricks with flawless play all around) to 11-trick to 12-trick to 13, the frequency tends to fall off precipitously. I don't have the figures and don't even know if anyone has worked them out, but I know it is thus. So you want to look to that 10-trick game if at all feasible as the big boys do.
Partials and slams also offer more points, the former being of particular interest to duplicate players, where 2 hearts is as good as 3 diamonds, saving a level, and 3 hearts is more than four diamonds, as mentioned above. But I would offer one caveat about slams: they are so valuable in themselves that if there is any doubt about the major as opposed to confidence in the minor, you'd probably do well to opt for the minor. But that's too vague to offer much help in specific situations, so on to some examples:

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

In either a spade or a club contract, you're going to lose a trick in each red suit and so it's apparent that in a club game you'd have to pick up both black queens, while in a spade game you can get by picking up only one. And that can make -- and indeed will make -- a big difference in your score over the long run. Indeed, with bad breaks or entry problems, there may be times where you finesse correctly for the queen and still lose a trick in the suit. So you'd do well to go for the 10-trick game.
You don't know where your partner's points are going to lie during the bidding. Put the points represented by the diamond honors into the two black queens, and you'd doubtless make five in either contract, a difference that is still important in matchpoint scoring, but rather trivial in other types of scoring. But then, your partner's points aren't always going to be their productive best, and you'd do well to go for the 10-trick game when you have both a major and a minor.

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

This doesn't look like an odds-on game bid. Declarer will probably have to lose 4 tricks in spades. Still, the major suit will serve far better as trump. First of all, for the matchpoint player, the spade partial will generate more points than a club partial, and in matchpoint scoring, of course, those small differentials can mean a decided advantage.
Further, if N-S should be feeling their oats and overbid to game, they would have some chance in spades. I guess we've all been in worse contracts, and not too improbable conditions (say E has A Q tight in spades) would allow game. But there would be no chance of making game in clubs, and an overbid to game would be to no purpose whatsoever. In clubs, even with a favorable positioning of the opponents' honors, you've always got to lose a heart, a spade and a diamond.

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

Notwithstanding the far stronger diamonds, this hand would fare far better in the major suit than in the minor. For starters, if West leads his singleton spade against a 5 diamond contract, the defense can take three quick tricks and beat declarer before he's even out of the starting gate. But even without a spade lead, this declarer was put to guess for the trump queen in five diamonds, and guessing wrong, went down.
Four spades is not for the careless, but you should make it. On a side-suit opening lead, you might consider a crossruff. You have lovely trump spots down to the six from the queen, and as for the A K, well, you're not in slam, and their absence was accounted for in the bidding. You could on, say, a heart lead, think of cashing the top two cards in every side-suit (sluffing two diamonds on the hearts, a diamond on the second club), ending in dummy. That's six tricks. Let's see if we can get four more on those 8 trump:
Ruff a heart, ruff a club, lead a heart. I say "lead" because East could ruff at this point. A low trump would simply be overruffed, causing you no inconvenience. He might ruff with a high trump (you sluff a club), cash his other high honor, then lead a trump to inhibit the crossruff. But the second trump lead is your 9th winner, and you can now crossruff for the last two trickks! So that wouldn't hurt you, and we'll presume East doesn't make that mistake.
So we ruff the second heart and that's nine tricks. Feeling a little more comfortable now? You ruff a club, which East overruffs and then cashes his second trump honor and then leads trump, which is your 10th trick and East gets the last trick with a trump.
Even if West gets off to a trump lead, which prompts East to cash his two high trump and lead a trump, you'd do all right. That plays havoc with a crossruff but doesn't queer the hand, because now we're going to make more use of that diamond suit. Let's count our winners. We've got a high trump in each hand and we want to be sure to ruff in one and draw the last trump with the other. That gives us 3 spade winners (we won the third trick, remember), two clubs and two hearts for 7, so we only need 3 diamond winners.
Win the third round of trump in dummy. Take the diamond hook into the hand that can't lead trump. Even if loses, you should have enough communication to get a ruff and then get to the other hand, draw the last trump. In this case, however, you're in luck, the finesse holds, and you have a play for 5, which is even better than the crossruff.
The argument here wasn't that the contract was cold against any distribution. In my first draft, I went through a discussion of a 4-1 diamond split, pointing out how it might queer the contract and then asked the rhetorical question: But would you rather be on level higher in diamonds on that break? The argument, rather, has been that the major suit will serve you better than the minor.
A novice player must bear this in mind:you don't avoid losers in a balanced fit by skirting it as trump. (Here you lose two spades in any contract.) And you don't lose the value of high cards by making it a side suit. (Here you still get the value of that diamond suit in a spade contract.)