This is the contracts package for StatsN and its sync counterpart.

This package contains the basic interfaces/Contracts that make up StatsN.

there are 3 interfaces. IStatsdSync which contains the contract for making sync calls to log stats, IStatsdAsync which inherits from IStatsdSync and contains the interfaces for any async calls. There is also IStatsd which inherits from both, and is just a shorthand for IStatsdAsync.

Which interface should I use?

Try to use IStatsd and use the async functions. Only use IStatsdSync if you plan on using the completely no async client.