While there is issue to be taken with the money=speech principle, Citizens United is, in fact, different. It is a direct attack on the principle of popular sovereignty: that the source of governmental power is popular consent. Corporations, which are endowed with power by governments, are by the Citizens United decision placed beyond regulation by the government. That means that the people, exercising popular sovereignty, created the corporate form through their government, but are disallowed to regulate it once created, placing corporate behavior outside the reach of the people. No more popular sovereignty.
How much money individuals qua individuals may spend, i.e., whether limits on individual monetary donations is a speech regulation, is an entirely different issue.