Which suit should be trump? Which suit indeed offers slam? Or are they pretty much equal? The category should tell you the answer to that. If the ace of clubs were onsides, they'd be equal in potential. If hearts split 4-1, they'd be pretty equal, though actually you'd have a peck of trouble on a diamond opening lead trying to test for the K of clubs, and spades would at least be safer. But even here, with an offsides A of clubs, it wouldn't matter. You'd have 11 top tricks and out. But on even breaks, and by the chance of an offsides A of clubs, then it does matter, and indeed would matter a great deal if you were in slam. [To be sure, the A of clubs opening lead and continuation would bring the two suits to equality in 11 tricks. I suppose I failed to mention that when this was first entered because I was discussing from the standpoint of the K of diamonds opening lead.]
In a spade contract, you'd have six spades, four hearts and a diamond and out for 11 tricks. But in a heart contract, you'll get an extra trick on a heart ruff in each hand for five heart winners. After drawing three rounds of trump, you run your spades, sluffing four clubs, and now lose a club. You would then be able to ruff a diamond in dummy and a club in the closed hand. Or at least you should.
But here's a declarer who made only 11 tricks in hearts, and no, he wasn't playing from the opposite hand. Let's see how come: Win the opening lead, take three rounds of trump. So far, so good. Run spades, sluffing a diamond and three clubs! A diamond and three clubs! ! ! And that diamond was his first sluff. Oh, please! That ittle-bitty deuce of diamonds is substantially a winner! No, it doesn't win a trick, but it's the card that allows you to pick up five heart winners. Without that deuce, you don't get a fifth trump winner. Plain and simple.
And it was costly. This declarer got 31% MP's for an overtrick, while those with 2 overtricks . . . oh, me, I thought I'd jotted that down, but I know it was in the neighborhood of 70.
It's costly when overtricks are easily garnered. Very costly. What does a winner look like? I have asked elsewhere. Well, here it looks like the deuce of diamonds . . . in conjunction with a heart opposite. (In spades, you can ruff also, but it's in the long hand and is no different from leading a spade, sluffing the low diamond.) And incidentally, the last trick involved a bit of overkill as the J and 10 of hearts were both played. One of them should have been ruffing a diamond.
Please note that these powerful solid suits can work just as effectively or more effectively as side suits. Now that presupposes an adequate trump elsewhere and, well, we're talking about the two majors or the two minors. (If a minor vs. a major, I'd go for the latter.) Here the spade suit is going to bring home 6 tricks whichever suit is trump. But in hearts, it's also going to sluff clubs, allowing an extra trick in hearts. The 12th winner is a heart, but it's the sixth spade that makes that possible.