From the text:
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. (Chapter Eight)
Outside of their love for Catherine, Edgar and Heathcliff are very different both physically and emotionally. Heathcliff is dark, temperamental, harsh, and easy to anger, as opposed to Edgar, who is easy going, slow to anger, fair haired and blue eyed.