To Mr. Collins's proposal Elizabeth react this way:

To such perseverance in wilful self-deception Elizabeth would make no reply, and immediately and in silence withdrew; determined, if he persisted in considering her repeated refusals as flattering encouragement, to apply to her father, whose negative might be uttered in such a manner as must be decisive, and whose behaviour at least could not be mistaken for the affectation and coquetry of an elegant female. (End of Chapter 19)

To Mr. Darcy's proposal she reacts this way:

The tumult of her mind was now painfully great. She knew not how to support herself, and from actual weakness sat down and cried for half an hour. Her astonishment, as she reflected on what had passed, was increased by every review of it. That she should receive an offer of marriage from Mr. Darcy! that he should have been in love with her for so many months! -- so much in love as to wish to marry her in spite of all the objections which had made him prevent his friend's marrying her sister, and which must appear at least with equal force in his own case -- was almost incredible! -- it was gratifying to have inspired unconsciously so strong an affection. But his pride, his abominable pride -- his shameless avowal of what he had done with respect to Jane -- his unpardonable assurance in acknowledging, though he could not justify it, and the unfeeling manner in which he had mentioned Mr. Wickham, his cruelty towards whom he had not attempted to deny, soon overcame the pity which the consideration of his attachment had for a moment excited. She continued in very agitating reflections till the sound of Lady Catherine's carriage made her feel how unequal she was to encounter Charlotte's observation, and hurried her away to her room. (End of Chapter 34)

So why the difference if she really disliked him so much?