I am currently working on a start-up where we started with .NET running on Azure and SQL server with Android and iOS clients.
After going through some realistic usage patterns and the cost of licensing, we ended up deciding to switch over to MongoDB/Python and Linux VMs.
It was painful to do the switch and the time we lost was not insignificant. However, in our case the time lost was a fraction of the future operating costs so we bit the bullet.
The stack you know is the best stack to use. After that, the stack that costs you the least is the best one to use :)