The best choice always depends on your data and the quality of the implementations for the language you use. We tested many serializers recently for our backend (Perl, but we wanted something more open to other languages than Storable) and found msgpack slightly faster and 20-25% smaller than JSON at encoding, but only half as fast at decoding, so we chose JSON.

The most surprising result was that JSON was 2x (decoding) to 3x (encoding) faster than Storable. The downside is that it sets the utf-8 flag ...