Technically we use Newtonsoft and Jil, Jil replacing Newtonsoft as we become increasingly confident in it.

I wouldn't suggest anyone use Jil in a production role unless you're at Stack Overflow. It's too untested at the moment, and the typical person can't get me on the horn to fix whatever just broke.

Why would I use Jil over Newtosoft ?

You wouldn't right now (Kevin doesn't recommend it). But in the end it you'll want to use it if JSON serialization is a performance bottleneck for you.