What is the difference between Microdata or Schema.org (or can I use both)?
They are different things: 


Microdata is a syntax. 
Schema.org is a vocabulary. 


If you want to use the Schema.org vocabulary, you can use the syntax Microdata, or RDFa (Lite), or JSON-LD (or any other RDF serialization, but the three mentioned syntaxes are supported by the big search engines that sponsor Schema.org).



The schemaorg module integrates the Schema.org vocabulary into Drupal 7 by using RDFa 1.0, which is the syntax used by Drupal 7 for other vocabularies, too.

The microdata integrates the Schema.org vocabulary by using Microdata (without this module, Drupal 7 does not output any Microdata).

If you can’t decide between these two modules: Go with schemaorg, because Drupal 7 already uses RDFa, RDFa is more powerful than Microdata, and parts of this module were ported into the Drupal 8 core.