In case you are still wondering on this ....
Choice between SWF vs jBPM - really depends on your application use case.
That said, here's a quick overview in layman terms
Complexity - jBPM is more complex and robust than SWF
Reliability - Application reliability is more based on how you write your application and has less to do with which framework you choose. So, this has no bearing on what you choose.
Cost - Well, similar to reliability, very subjective. jBPM is open source workflow engine but once you build your application you would have to run into AWS and that cost depends on how many instances, how much throughput, etc, etc.
Maintenance - I think AWS SWF would be easier to maintain only because its a simpler framework with limited functionality.
I think you start with your application requirements and from there, you understand what kind of capabilities you need and then decide.
For example, if you don't need robust workflow and business process in your application, then you can go with AWS SWF.
Also, your preferred programming language should be considered as well - Java (for jBPM) vs Java, Php, Ruby, .NET,etc (for SWF).