Recently I was pointed to the so-called Oracle Learning Streams http://education.oracle.com/streams which provide short presentations on all kind of topics.
While ironing my clothes on a Sunday afternoon, I watched one with the title "Leveraging OBPM vs BPEL" by David Mills. An excellent story where he explains in less than 13 minutes the high-level difference using a practical example.
One reason why I like about this stream is that it is in line with what I preach for years already. Otherwise I would have told you it sucked, obviously.
The main point David makes is that you should use the right tool for the right job. OBPM aims at orchestrating business functions, whereas BPEL aims at orchestrating system functions. The example used is an orchestration of system functions to compose an Update Customer Profile service, which then can be used in a business process, orchestrating business functions where one person is involved to approve some update, while someone else needs to be informed about that. Watch, and you'll see!
For understandable reasons the presentation does not touch the (technical) details. Without any intention to explain those, one should think about differences in the language itself (for example in BPEL you cannot create loops while in BPMN that quite normal to do), and also in the area of configuration and tuning (for example in case of BPEL there are more threads to tune, and you can do in-memory optimization, etc.).
Maybe I find some time to give you a more detailed insight in those more detailed differences. Would help if you would express your interest by leaving a comment!