Go with BPEL for now and target the same version and implementation we use for the ESB :-)
I think going with the BPEL initially makes sense, although one concern here is that from a modeling perspective I don't know that the AE BPEL Designer is available on Eclipse 3.4 yet. and the latest JBoss and PI4SOA tooling is targeted at Eclipse 3.4.
As for jBPM, well the ESB Service Node does sort of support message types; that is there is an Input and Output variable mapping where the user can define the mapping of jBPM context variables to ESB Messages. This is more for mapping of Java objects, although it should support XML as well. However, once having mapping XML data into jBPM, jBPM is not able to work with XML data (can only store the string), and there is no equivalent of a BPEL "assign" to convert the XML into variables that could be used (say for conditional branching at a decision node).