I think you'll find it difficult to compare and contrast 2 fairly different Technology Stacks with different goals/focuses. Never having used NServiceBus I wont be able to comment on its Features/Strengths - I invite you to evaluate both to determine which is the most suitable for your use-case and which one you enjoy developing with more - given they have largely different feature-sets, another option to consider would be using both, complimentary together.
ServiceStack Core Goals and Features
As the project lead for ServiceStack I can only speak to ServiceStack's core goals/focus which aims to simplify development and maximize end-to-end productivity with a core focus on reducing artificial complexity:
Reduce Artificial Complexity and Maximize Utility
One of the ways it reduces cognitive load is by promoting a single flexible and versatile approach to developing Services, promoting remote services best-practices
with a message-based design by simply designing your Services around accepting and returning clean POCO Data Transfer Objects (DTO's) (but can also return any custom media type or binary/image responses).
By using well-defined POCOs to define your Services Contracts, ServiceStack is able to infer a greater intelligence about your Services and automatically provide a range of Metadata features including built-in Metadata Pages with links to XSD's, WSDLs and Plugins like Postman and Swagger UI Support. ServiceStack's also able to enable RDBMS-driven Services with AutoQuery from just a Request DTO definition which is how Stack API's can provide all its Search functionality, using a single Request DTO.
End-to-End productivity
Having a clean separation of DTO's from your Service implementations is what allows you to be able to share your server DTO's on the client for an instant end-to-end typed API without code-gen. Alternatively if preferred, clients can, with just a url,
Add a remote ServiceStack Reference to generate typed DTO's for
C#,
F#,
VB.NET
and
TypeScript
clients.
Maximize Re-use
ServiceStack is able to maximize the re-use and utility of your Services where ServiceStack Services are able to be consumed via an array of built-in fast data formats (inc.
JSON,
XML,
CSV,
JSV,
ProtoBuf and
MsgPack)
as well as SOAP endpoints and MQ Hosts.
Your same Services also serve as the Controller in ServiceStack's Smart Razor Views
reducing the effort to serve both
Web and Single Page Apps as well as
Rich Desktop and Mobile Clients.
Messaging Features
For Messaging Features, ServiceStack provides a clean Messaging API where your Services can be consumed via Rabbit MQ, Redis MQ and In Memory MQ hosts. Although as the Messaging API re-uses your existing Services it follows the same Request/Reply MQ pattern.
Separate to the Messaging API, ServiceStack provides a Redis Pub/Sub Server library which is what powers Redis Server Events and Redis MQ. The Pub/Sub features also extend to ServiceStack's Server Events which provides real-time features to Ajax/JavaScript and .NET ServerEvents Clients allowing you to easily build rich, live Web Apps like React Chat and jQuery Chat (in less that 200 lines of JS).