"Two offences of a very different nature, and by no means of equal magnitude, you last night laid to my charge.

Of Elizabeth's accusations, Darcy obviously feels that greater offense was the "ruination" of Wickham. Look at how he writes about the 2 offenses.

The first-mentioned was that, regardless of the sentiments of either, I had detached Mr. Bingley from your sister ... the separation of two young persons, whose affection could be the growth of only a few weeks, could bear no comparison [to Wickham's claims].

He states this in a straight forward manner, rather off-hand.

and the other, that I had, in defiance of various claims, in defiance of honour and humanity, ruined the immediate prosperity and blasted the prospects of Mr. Wickham. -- Wilfully and wantonly to have thrown off the companion of my youth, the acknowledged favourite of my father, a young man who had scarcely any other dependence than on our patronage, and who had been brought up to expect its exertion, would be a depravity,

He becomes quite verbose in regards to Wickham. I believe that is because the business with Wickham is much more personal to him than the love life of his friend. He can be detached with Bingley, he cannot be detached with Wickham.

[Wickham] so far recommended himself to Georgiana... that she was persuaded to believe herself in love, and to consent to an elopement. ... Mr. Wickham's chief object was unquestionably my sister's fortune, which is thirty thousand pounds; but I cannot help supposing that the hope of revenging himself on me was a strong inducement. His revenge would have been complete indeed.

Darcy feels the Wickham is more important because it involves his sister, not realizing his interference with Jane & Bingley was most likely places more importance to Elizabeth because it is her sister.