Best practice to log both or one of these? (1) "HTTP_X_FORWARDED_FOR" (2) "REMOTE_ADDR"

Question

I modified the code from here below: How to Get IP Address?

static string IPForwarded =
  string.IsNullOrEmpty(HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"])
  ? null : HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"].Split(',').Last().Trim(); 

static string IPRemoteAddress = 
  string.IsNullOrEmpty(HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"])
  ? null : HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];

// Returns a simple message with both IPs
private static string GetIPMessage() { return 
    (String.IsNullOrWhiteSpace(IPForwarded) ? "" : "[HTTP_XFF " + IPForwarded + "] ") + 
    (String.IsNullOrWhiteSpace(IPRemoteAddress) ? "" : "[REMOTE_ADDR " + IPRemoteAddress + "] ");
}

In most variations of the original code, it returns the last IP in the list HTTP_X_FORWARDED_FOR if it isn't empty. Or, it returns REMOTE_ADDR. Is there any case where I should log both (as I do in GETIPMessage() for possible IP blocking? In other words, are they mutually exclusive? Would one variable always be empty or useless if the other is present?