Visual Studio Online. Not sure given that MSFT is giving away Bizspark access to Visual Studio for startups that anyone would even start with tracking them in Google Spreadsheets.
Your team is going to be (or should be) developing in Eclipse or Visual Studio. So your code should reside in either GitHub or Visual Studio Online - regardless of the platform you will be executing the code on.
Both GitHub and Visual Studio Online have Agile, Scrum and Waterfall workflows built in.
Personally I am a great fan of Visual Studio but I understand that this is more of a 'what you grew up with' issue. But it is a great mistake not to be using a source code repository with formal check in and out requirements even for small projects. Why? Because you need a way to roll back to a known good state... always