Which metadata endpoint should I prefer for creating object / model in CRM?

Hi,

I am working on integration project of MS Dynamics CRM online version. In WEB APIs I have observed below APIs are available which gives the complete information about the object metadata (fields/datatype/descriptions) for example Account, Contact, etc:

1. XYZ.com/api/data/v8.2/$metadata  (returns XML CSDL data)

2. XYZ.com/api/data/v8.2/EntityDefinitions (JSON data)

Would like to know,

1. Which endpoint should I use for creating object/ creating schema ?

2. Why NavigationProperty are missing from EntityDefinitions? example, for Account object NavigationProperty 'primarycontactid' is present in CSDL doc (endpoint #1) but the same field is missing from EntityDefinitions (JSON metadata).

Thanks