In the karma system, users are given points according to other user's rating on their comments, and the more karma you have, more karma people rated by you will gain. It's a reliable measure of someone's popularity, by community itself.
In the score system you earn point's by your actions or other user's actions related to you, like reading an article, writing a message, creating a wiki page, having your topic replied in forum, etc. This is a quantitative measure, not by community but by computer.
|Points according to other users rating on published content
|Points for every action, including other users rating
|Reflects user popularity
|Reflects user activity