To quickly gain momentum, a series of small and useful tools always trumps a monolithic, inappropriate, hard to use tool.
People are familiar with certain tools already -- give the people what they want.
The choice here does not need to be open source. If you choose a non-open solution, you incur additional risk for whatever opportunity you are trying to capture. Make sure that a move to fully open tools is part of your roadmap for the project.
Some people will show up to participate no matter what tool you choose. Another group will participate only if the tool is open source, with some preferring popular tools. Why not choose an open, popular solution and capture all groups?