XPDL is more flexible than BPEL as I know, (and If you want to use open  source workflow  engines) as I know there are more open source  workflow engines that work on Wfmc standard  than BPEL standard.
I think XPDL model is more complex than BPEL.
And about Web Service I am not sure that XPDL or WFMC based engines don't work with Web Service. I know that Wfmc provides a sophisticated protocol in Its 4 or 5th interface which is dedicated to Exchanging Process with Other workflow engines by means of Web Services.