Which is better for mobile application xml or json

I am writing a web service in java that will return some data. This service will be consumed by mobile application (ios and android). I checked several posts like this and also googled in general. Most posts talk about json vs xml but I an not sure how much of these are applicable in mobile environment and want to know the pros and cons of xml and json when consumed from a mobile app. And what are the factors that should be kept in mind when deciding the format like that amount of data, contents of data etc.