I think it is worse than you suggest. The Democrats and the Republicans receive funds from the same interest groups, companies and individuals. They preside over a political system that is deliberately bent to ensure that competition has little or no chance. They cooperate in this carve up because it suits them both.
I think Nader would suggest voting for people who represent your interests, be they Rep- Dem or others. It is a severe case of the FF: FG syndrome. The voters could easily change that, but they don't understand that they do have that power and that the 'them or us' equation as presented is not necessarily the only equation.