Capitalism provides an opportunity for people to prosper and allows them to grow rich. However, it also divides the society into 'have's and 'have not' s.
Democracy allows participation of everyone and ensures the interests of everyone.
A combination of these systems (as seen in United States) offers certain advantages and disadvantages.
A capitalist democracy will have a number of big businesses and a large number of rich people. The presence of big revenue and in turn taxes will allow the democracy to spend on facilities and services that can be used by all. A socialist democracy suffers from lack of resources, which should not be as big a problem here. A democracy allows stability in administration, which in turn helps entrepreneurs grow and take risks. And since democracy aims to provide education to all, a large workforce is available for entrepreneurs to hire (even if it is a selective process).
However, rich people may not want to pay for those who can't. Simply put, taxes paid by rich people will be used to build infrastructure (parks, bridges, roads, etc) and services (such as medical care) and even the people who can't pay taxes use them. This system works by imposing high tax rates on rich individuals and businesses. Capitalist democracy also suffers from frequent government intervention in the market. This sometimes leads to businesses bribing the politicians and introduces corruption in the system.