Apex REST web service is the optimal method to implement this in Salesforce because it allows you to expose your Apex classes and methods as RESTful web services that can be accessed by external applications. Apex REST web service can also return data in JSON format, which is suitable for the native iOS app. Apex SOAP web service, on the other hand, uses XML format, which is more verbose and complex to parse. Apex SOAP callout and Apex REST callout are used to invoke external web services from Apex, not to expose Apex as web services. Reference: [Apex REST Web Services], [Apex SOAP Web Services], [Apex Web Services and Callouts]