You are confusing vocabularies with syntaxes/formats:
- Schema.org is a vocabulary.
- RDFa and Microdata are syntaxes/formats (no vocabularies attached)
- Microformats is both, a syntax coupled with a specific set of vocabularies.
You may use the Schema.org vocabulary with various syntaxes, like RDFa, Microdata, JSON-LD, Turtle etc.
Different consumers (services like search engines etc.; agents like browser add-ons etc.) support different vocabularies and different syntaxes. If you are interested in only one or a few specific consumers, check their documentation and see what they support. Of course it may be the case that consumer A only supports Microdata and consumer B only supports RDFa.
If you like to support as many consumers as possible (i.e., even those consumers that you’ve never heard of or that will come up in the future), you may consider using several syntaxes in the same document.
For the consumer Google Search in particular: Google claims to support three syntaxes for Schema.org:
So if you care only about Google, there should be no reason to use more than one of these.