How do I compare the use of words in Dickens' Great Expectations and Emily Brontë's Wuthering Heights?

This is a broad question. I can give you a start on the beginning basic steps in word usage analysis, then you can progress from there. The word usage in the first paragraphs of both works show irony. Both stories are told by first-person narrators, so it is the narrators who use irony either in narratorial passages or in dialogue. The striking difference between Brontë's and Dickens' kinds of irony is that Dickens' is humorous while Brontë's is bitter.

DICKENS
Humorous Irony: "The shape of the letters on my father's, gave me an odd idea that he was a square, stout, dark man, with curly black hair."
BRONTE
Bitter Irony: "The ‘walk in’ was uttered with closed teeth, and expressed the sentiment, ‘Go to the Deuce.'"

The humor in Dickens' is needed to help build sympathy for Pip as he proceeds to make one life mistake after another as the story progresses, thus creating a morass that is symbolized by the dark threats of the moor. The bitterness in Brontë's is needed to develop the dark tone of enmity and psychotic selfishness surrounding both Heathcliff and Catherine, psychological character aspects symbolized by the moor.

Vocabulary and syntax choices are important word usages. For instance, Brontë and Dickens both make use of adjectives and adverbs, yet Brontë emphasizes adjectives, while Dickens emphasizes adverbs and uses fewer superlative adjectives.

DICKENS:
Adjectives: "exceedingly early in that universal struggle"
Adverbs: "were unreasonably derived from their tombstones"; "gave up trying to get a living, exceedingly early"
BRONTE:
Adjectives: "This is certainly a beautiful country!"; "A capital fellow!"; "A perfect misanthropist’s heaven
Adverbs: "completely removed from the stir of society"

The word usage choices for tense are similarly reversed and interestingly different. Dickens starts out with present and past tense, then switches to past tense in perfective aspect (past perfective) where needed. Brontë starts out with past perfective word usage then switches to past tense. One might ask: Should this be analyzed as clumsiness in her craft or as something done by intent? The opening sentence might easily be paraphrased in past tense to give it the same immediacy that Dickens' narrative has, like this: "I just returned .. I see I fixed on the perfect ...."

DICKENS:
Present and Past Tense: "my infant tongue could make of both names nothing longer or more explicit than Pip. So, I called myself Pip, and came to be called Pip."; "I give Pirrip as my father's family name,"
Past Perfective: "A man who had been soaked in water"
BRONTE
Past Perfective: "I have just returned"; "that I could have fixed"
Past Tense: "how my heart warmed towards him"; "When he saw my horse’s breast fairly pushing the barrier,"

Finally, both use dialect and do it with equal skill while rendering the dialect of their individual settings.

DIALECT
Dickens: "Who d'ye live with,--supposin' you're kindly let to live,"
Brontë: "‘T’ maister’s down i’ t’ fowld.  Go round by th’ end o’ t’ laith".