A mixed economy cannot be as good as a capitalist economy. I'm not much of an expert on economic systems but one thing I've noticed is that the European Union and the International Monetary Fund always encourage governments to adapt a capitalist economic system to help their economy grow and for the country to become wealthier.
One of the requirements of the International Monetary Fund, for example, is for governments to privatize state owned businesses. Private businesses are said to be more efficient and profitable and so are better for the economy.
In a mixed economy, businesses will continue to be owned or controlled by the government which means less profit and less wealth for the country.