IMO, Microsoft Orleans is a framework and an implementation for developing highly distributed concurrent applications, which can also be considered as a wrapper or a developer friendliness (or the so called developer productivity) coated actor framework.
Microsoft Orleans said to be having the concept/notion of virtual actors and optimized for cloud. As in production Orleans is deployed in Azure cloud services and on premise deployment. IMO – The production implementation of the Orleans is quite challenging compared to Akka.NET implementations.
The idea behind the developer friendliness is quite confusing; the so called developer friendliness is achieved through abstracting and hiding plenty of underlying concepts of actors model and some areas Orleans has key breaches to the actor model as well. In that sense one can argue that Orleans is not an actor framework. So if you’re a person who is into details, you might find it bit less involved, but if you’re a developer who really want to create a quick solution for a burning business issue this is ok.
I highly recommend, you to read actor model and how it works and then get into Orleans as this would give clear picture of Orleans and how it is implemented.
If I’m to tell the fundamental difference between Orleans and Akka.NET based on the developer learning aspect, it is same as the exact difference between Java and C#. Java is pure (recent versions are quite different, if you have used Java 1.5/1.6 you’d understand) on object orient programming and a good tool to learn the real concepts of OOP. But C# has OOP features but not a strict follower of it, on top of the OOP concepts it goes beyond the OOP language constructs in order to achieve developer friendliness and productivity.
Orleans is a derived innovation on top of the actor model concept and Akka.NET is more over a real mapped implementation of actor model.