Nelly's narration uses landscape imagery to portray Heathcliff and Edgar Linton as foils for one another, particularly to show that Heathcliff (despite being of "foreign" origin) is very much of the moors, while Edgar belongs to a gentler, more refined world. She makes this comparison on the day that Edgar ultimately proposes to Catherine, and Heathcliff leaves the Heights:
Doubtless Catherine marked the difference between her friends as one came in, and the other went out. The contrast resembled what you see in exchanging a bleak, hilly, coal country for a beautiful fertile valley; and his voice and greeting were as opposite as his aspect—He had a sweet, low manner of speaking, and pronounced his words as you do, that’s less gruff than we talk here, and softer.
"Bleak, hilly, coal country" refers to Heathcliff; Nelly associates him with the Yorkshire moors. This is especially interesting because Heathcliff first came to Wuthering Heights from the streets of Liverpool and is called a "Gypsy" more than once (a member of the nomadic Romani people). Whether that term is meant literally or not, Heathcliff definitely isn't from the Yorkshire moors originally—yet his dark complexion and dark affect make him appear well suited to the coal country landscape.
On the other hand, "beautiful fertile valley" and "sweet, low manner of speaking" refer to Edgar Linton, associating him with a more "civilized," gentleman's way of life. Even though Edgar grew up at Thrushcross Grange and therefore belongs to the moors in a way that Heathcliff doesn't, he doesn't resemble them at all. Nelly even points out that Edgar has a softer way of speaking that resembles an outsider's, like Lockwood's. The words "fertile," "sweet," and "soft" even suggest that Edgar is more feminine than Heathcliff.
By setting up Heathcliff and Edgar as foils, Nelly brings out the stark differences between the two young men, but she also highlights the fact that both these sets of traits appeal to Catherine—heightening the tension around the choice Catherine will imminently have to make. Her particular use of nature imagery also suggests that someone's birthplace doesn't determine their character: Heathcliff has been allowed to run wild on his adopted moors, while Edgar has been sheltered at his native Grange, leading to very different personalities.
Nelly wonders aloud why Edgar Linton and Hindley Earnshaw—both loving husbands and fathers initially—took such different paths in life, presenting them as foils for each other.
[...] Hindley, with apparently the stronger head, has shown himself sadly the worse and the weaker man. When his ship struck, the captain abandoned his post; and the crew, instead of trying to save her, rushed into riot, and confusion, leaving no hope for their luckless vessel. Linton, on the contrary, displayed the true courage of a loyal and faithful soul: he trusted God; and God comforted him. One hoped, and the other despaired: they chose their own lots, and were righteously doomed to endure them.
Nelly describes Hindley as an outwardly more robust man, yet when his "ship struck"—a metaphor referring to tragedy, in this case the death of his wife—the "captain abandoned his post," and the "crew" allowed the ship to founder, seemingly referring to Hindley's reason and other faculties. Instead of standing up in the face of disaster, in other words, Hindley gave in to a life of drunken dissipation instead of making wiser choices—essentially letting tragedy shipwreck his existence.
On the other hand, Linton, seemingly a softer and weaker man, suffered a comparable tragedy when Catherine died, yet proved himself the stronger man—he simply kept living with courage and faith as best he could despite his grief. From Nelly's perspective, then, hope and faith in God are evidence of the kind of strength that really counts—not mere outward strength. Linton's strength throws Hindley's weakness and wasted life into sharp relief, and even suggests that a more "feminine" man like Linton can be stronger than a more conventionally masculine man who misuses his resources.
Note, finally, that Nelly uses "doomed" in a more archaic sense here—i.e. "fated" to endure something, not necessarily "cursed" to do so—and also suggests that each man's choices helped shape the "doom" that awaited him, not fate alone.
Linton and Cathy are comparing their ideas of heaven's happiness and get into a quarrel about it. Their incompatible imagery also sets them up as foils for one another.
He said the pleasantest manner of spending a hot July day was lying from morning till evening on a bank of heath in the middle of the moors, with the bees humming dreamily about among the bloom [...] mine was rocking in a rustling green tree, with a west wind blowing, and bright, white clouds flitting rapidly above [...] and the moors seen at a distance, broken into cool dusky dells [...] and the whole world awake and wild with joy. He wanted all to lie in an ecstacy of peace; I wanted all to sparkle, and dance in a glorious jubilee.
Cathy goes on to say disdainfully that Linton's heaven would be "only half alive," and he retorts that Cathy's would be "drunk."
Linton's image of "bees humming dreamily" among fragrant blossoms is drowsy and hypnotic, while Cathy's image of rocking in the branches of a windblown tree is bright, bracing, and pulsing with life. Notably, Cathy's imagery is more closely akin to the moor itself—wild and untamed. Like her mother Catherine, Cathy finds an expression of her own soul in the "sparkle" and "dance" of nature.
But where Cathy is lively, adventurous, and keen, Linton is weak, unambitious, and content to dream his days away. Though his ideal is also described in terms of nature, it's much more sedentary than Cathy's. The contrast in their ideas of heaven cements what's already apparent—that they're a poor match for each other. They're companions because Heathcliff orchestrated the friendship, but Linton mostly enjoys being doted on by Cathy, and Cathy mostly enjoys babying Linton and bossing him around. If their ideal heavens are so incompatible, then it's hard to see how they would thrive as husband and wife in everyday life on Earth.