I am working to make an survey to choose a workflow  engine for our
application hesitate to choose an standard  language BPEL vs XPDL
as you know BPEL is suitable for web services and used widely in work
by
famous corps like Microsoft , Oracle , SAP , IBM , sun , .. ..
but Xpdl is standardized by WFMC and OMG ?
is anybody expert in workflow engines that can recommend  good
criteria for choosing a best alternatives ?
what about an open  source implementation in java ?

Take a look @ here:
www.java-source.net/open-source/workflow-engines

I myself was interested in the field. As SOA seems to become epidemic and
every one is going toward it, I myself chose BPEL, although WFMC seems to be
more complete.

you can using GNU/ActiveBPEL and if you using application server JBoss you can use JBPM that stand on BPEL Standard.

Just forget BPEL, I think we should wait till 2.0 version of it's specification , as you know current version does not support human interaction .
and also it is important to note that BPEL usage is for webservice orchestration.
So my suggestion is to go with workflow  engines that implement WFMC standard  like :EnhydraShark OR none standards ones like : JBPM and openWFE .
if I were in your shoes , I would consider JBPM .

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.