After writing the last piece it struck me that I had unintentionally suggested that BPEL and XPDL are somehow competing for the same role, which just isn't right. This post is an excellent summary of the true position - roughly, XPDL for process description, BPEL for programming.
So what I think I'm most interested in is using a combination of these two, treating the design of a workflow-driven solution as a layered artifact. The XPDL is relatively technology-neutral: the description is an abstraction which can be shared across an enterprise (or even between enterprises) because it doesn't commit to implementation details.
The XPDL layer would then be used to drive the BPEL layer, where processes and activities which appear as abstractions at the XPDL layer are translated into concrete actions (and transactions) against real systems or service endpoints.
When I get some time (!) I plan to experiment with this combination, to see how practical it is for modelling an existing healthcare workflow I've been involved in.