Google recommends using microdata, but it does support three formats: microdata, microformats, and RDFa. A big reason to choose microdata would be that the examples that Google gives on it's website and those on schema.org are in the microdata format.
Here is a site that has a huge table of the various advantages and disadvantages of the three formats. The items that I find most compelling on the table are "relative complexity", "target languages", and "new attributes".
Microformat is the least complex, works with XHTML1, HTML4, HTML5, and XHTML5, and does not add any additional tag attributes into your document. You do have to rename some of your html class names and change your CSS to support it.
Microdata is slightly more complex and has some additional capabilities, but only works with HTML5, and XHTML5. It gets around having to rename some HTML classes and change CSS by addition a few additional tag attributes.
RDFa is even more complex, but adds even more additional capabilities and works with a large number of doctypes.