Interest groups have many resources (size, money, information) at their disposal. Which is the most effective resource?
This depends on the nature of the issue at hand.
When it comes to issues that are easily understood by the public and which have some emotional value, the most important resource is size and the fervor with which beliefs are held. On issues like abortion or gun control or even the basic idea of universal health care (as opposed to the details), an interest group with many motivated members and little money will defeat one that has money but few members.
However, when the issue is detailed and complicated and hard to understand, it is the interest groups that have money that will win. These groups can give campaign donations and can hire lobbyists to persuade members of Congress to take their point of view. The public will not be able to really get motivated on the issue because it's too hard to understand. An example might be with the regulation of banks or of the financial industry and their use of derivatives. This kind of thing is too hard to understand and so the public will not really rise up on one side of the issue or the other. In such a case, money and the access that it buys becomes the most important and effective resource.