Which do you think is better design? (in the perspective of scalability, consistency, etc.)

design.swift

////////////////////////////////////////////////////////////////////////////////
// Usage                                                                      //
////////////////////////////////////////////////////////////////////////////////

struct JoinForm {
  var emailField: FormField
  var usernameField: FormField
  
  init(email: String?) {
    self.emailField = FormField(value: email)
    self.usernameField = FormField(value: parseUsernameFromEmail(email))
  }

  init(socialAccountProfile: SocialAccountProfile) {
    // autofill value from social account profile
    self.emailField = FormField(value: socialAccountProfile.email)
    self.usernameField = FormField(value: socialAccountProfile.username ?? parseUsernameFromEmail(socialAccountProfile.email))
  }
}

let socialAccountProfile: SocialAccountProfile = FacebookCredential.login() // can be replaced with TwitterCredential.login(), ....
let joinForm = JoinForm(socialAccountProfile: socialAccountProfile)
let joinFormViewController = JoinFormViewController(joinForm: joinForm)
present(joinFormViewController, animated: true, completion: nil)


////////////////////////////////////////////////////////////////////////////////
// Candidate A. 1 struct and extend with initializer                          //
////////////////////////////////////////////////////////////////////////////////

// SocialAccountProfile.swift
struct SocialAccountProfile {
  let email: String?
  let username: String?
}

// FacebookCredential.swift
extension SocialAccountProfile {
  init(facebookUser: [String: Any]) {
    self.email = facebookUser["email"] as? String
    self.username = facebookUser["username"] as? String
  }
}

// TwitterCredential.swift
extension SocialAccountProfile {
  init(twitterUser: TWTRUser) {
    self.email = nil
    self.username = twitterUser.screenName
  }
}


////////////////////////////////////////////////////////////////////////////////
// Candadate B. 1 protocol and N structs                                      //
////////////////////////////////////////////////////////////////////////////////

// SocialAccountProfile.swift
protocol SocialAccountProfile {
  var email: String? { get }
  var username: String? { get }
}

// FacebookCredential.swift
struct FacebookProfile: SocialAccountProfile {
  let email: String?
  let username: String?

  init(facebookUser: [String: Any]) {
    self.email = facebookUser["email"] as? String
    self.username = facebookUser["username"] as? String
  }
}

// TwitterCredential.swift
struct TwitterProfile: SocialAccountProfile {
  let email: String?
  let username: String?

  init(twitterUser: TWTRUser) {
    self.email = nil
    self.username = twitterUser.screenName
  }
}