I wonder if Lizzy judges Charlotte more harshly because she knows more of Mr Collins and she does not think very highly of him - a conceited narrow little man, as she calls him. So in her eyes Charlotte is quite clearly marrying for security, and not for a romantic reason, to a man Lizzy feels has little or no qualities that would make a sensible woman actually like him. Wickham she judges less partly because she likes him and has been flattered by his attentions - she's only human :) - and also because Miss King is a 'good sort of girl' - that is, she is not an objectionable match as Mr Collins is, she's just pleasant enough. If Miss King was also a very objectionable character, she might judge Wickham a little more harshly perhaps ??