What a Good Fit Can Do

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

North East SouthWest
Pass1 2 Pass
Pass 2 Allpass

The South hand (which didn't wind up as declarer) was bid by a novice, but that's of no importance here. What's important is to see that having two five-card suits makes this hand significantly stronger than, say, giving the queen of hearts to the spade suit for the same hcp's, but only one biddable suit. The heart suit isn't the sort of holding you want to insist on. But five pieces plus a second suit (and 17 hcp's) make that suit eminently biddable. You have a partner, and should be able to trust that partner to prefer the better suit.
Here, as the bidding shows, the opponents were able to steal a 2 spade bid, giving N-S a 25% score. Over one spade, South should bid two hearts, and over two spades, 3 diamonds and let her partner take his choice, which would be obvious. Without going through all the permutations here, it looks to me as though there'd be a good shot at 4 hearts. Declarer should cash two top diamonds and ruff one before drawing trump, losing (as I see it), one club, one heart and one diamond.
In any event, hearts would be much the better contract. You have to bear in mind two (related) truisms: If you have a fit in a suit without all top cards, you've got losers in it whether it's trump or not, and secondly, if you have high cards in a suit, you've got likely valuable winners there even if it's a side suit.
After having written that, I went to the record and found that just about everyone made 4 hearts. I checked the play of one declarer who didn't, and it was his misplay, not sterling defense that led to that result. And other declarers didn't have such an easy time in the bidding as this South player was offered. Over three spades, one doubled, then bid 4 diamonds over 4 clubs, leading the North player above to surmise, correctly, that this must be a two-suited hand and bid 4 hearts on the K J 10. Gutsy. Another (South) simply bid 4 hearts over 3 spades!
In any event, you don't wanna stick in the mud with the presumption that you wanna bid the stronger suit as trump. The length in hearts makes it as viable a suit for offering as trump (when you can offer two suits) as the diamonds. Your partner just might have the fillers if you'll give him/her a choice, as was the case here.