Android - Webservice，php vs. ASP.NET，XML vs. JSON？

I am in the process of developing my first Android app and I need to send/receive data from a remote MSSQL database. I am pretty comfortable with both php and asp.net so which one is the most efficient for interacting with android via webservice? and then what is the best/fastest means of communication, xml or json?I am in the process of developing my first Andr