Which one is better?

  def show(conn, _params) do
    invoice_id = get_session(conn, :invoice_id)
    invoice_access_token = get_session(conn, :access_token)
    cond 
      is_nil(invoice_id) or is_nil(invoice_access_token) -> render_error(conn, _params)
      _ -> render_form(conn, invoice_id, invoice_access_token)
    end
  end

  
  def w_nil(nil, k), do: {:error, {k, :nil}}
  def w_nil(v, k), do: {v, k}
  
  def show(conn, _params) do
    with {:invoice_id, iid} <- get_session(conn, :invoice_id) |> w_nil(:invoice_id),
         {:invoice_access_token, iat} <- get_session(conn, :access_token) |> w_nil(:invoice_access_token)
    do
      render_form(conn, iid, iat)
    else
      _ -> render_error(conn, _params)
    end
  end