I was checking my SEO score on few SEO sites and few of them told me that I don’t have Microdata or RDFa.
I searched and found this: Microdata vs RFDa
Good question, but I was wondering: Why should I use them? And which one to use?
You should use the format specified at schema.org. Google, Microsoft, and Yahoo have all agreed to use the formats listed there.
The syntaxes Microdata and RDFa allow you to add structured data to your documents.
Both use vocabularies for that purpose. You could create your own vocabulary, but it’s of course much more useful to use well-known vocabularies, because this allows more consumers to make use of your data.
And that’s the main reason for adding structured data: Allowing consumers (which could be search engine bots, clients, browser add-ons, CMS, etc.) to do something with your data. What this something could be depends on the consumer, of course.
A popular example is the vocabulary Schema.org. This project is sponsored by the search engine companies Google, Microsoft (i.e., Bing), Yahoo and Yandex, and they (as well as various other parties) use the data marked up with the Schema.org vocabulary for various purposes. Best known are their search result enhancements (e.g., "Rich Snippets" in case of Google Search).
You can see their differences in the question you linked to, Microdata vs RFDa.
You might want to check which syntax(es) the consumers you are interested in support. If all of them support both, you can choose whichever is easier to implement, you like more, or whatever.