No matter where you live, walled city or not, there is no Tahanun or LaMenatzeah on either day, you aren't allowed to fast on either day, and you can even fulfill Matanoth LaEvyonim on either day (according to some opinions, and subject to certain restrictions).
Clearly, both days are significant days to all of Klal Yisrael. Why, then, don't we all say 'Al HaNisim on both days?