MassTransit is lean service bus implementation for building loosely coupled applications using the .NET framework.
The lean implementation is supported by the YAGNI principle. By focusing on a tight set of specific concerns, the touch points between Mass Transit and the application are minimized resulting in a clear and concise set of interfaces.
Designed for collaboration between business-oriented services, NServiceBus is not a replacement for RPC technologies like WCF.
Successful SOA and DDD projects use a mix of approaches and technologies – not just NServiceBus for communications.
When many people hear the term "service bus" they picture a central box which all communication goes through, like BizTalk. That’s actually a description of the Broker architectural style, not the Bus architectural style. A bus isn’t necessarily a physical entity. In that respect, NServiceBus is more similar to WCF than it is to BizTalk.
There is no physical WCF one can point to in the network topology. WCF is part of the infrastructure that is run in-process with a given application’s code. NServiceBus is the same.
Just like you can write your own host process and activate WCF explicitly within it, you can do the same thing with NServiceBus. The bus in NServiceBus is something virtual – the collection of framework objects running in the various applicative processes.