XML has much better tooling (XSD, XSLT, xpath, etc.) but looks ugly and does not mix well with modern front-end technology which is all Javascript based. So I would go with JSON.
It's also worth mentioning that tooling and mapping can be very different depending on language. I've often found consuming services published in Java from another strongly typed language (c#) particularly painful, as the Java tooling makes certain Addington's towards itself instead of following spec.