There has been a lot of controversy about these formats recently, but the reality is both will be around for a long time. I don't think it is an either - or proposition.
Michael Scherotter at Mindjet Labs just posted about the ability for MindManager to export both OpenXML and ODF type documents. He takes a realistic and balanced view about XML formats in general. MindManager has been using an XML file format since 2003.
I'm a MindManager fan and have been using it for about that long.
When Mindjet built the integration with Word 2007 using the OpenXML file formats, it enabled me to send my mind maps around to many folks who don't have MindManager within Microsoft - as an Open XML Word document. OpenXML allowed them to do a high-fidelity round trip of the mind map which allows editing in both applications. It has also convinced more than a few folks to pick up a copy of MindManager 😉
It's very compelling example of what can be done now that XML file formats are becoming mainstream.