There is an interesting discrepency between the way the miracle of Purim is described and the way the miracle of Chanukah is described in our tefilah of "Al haNisim" . The recounting of the story of Purim in "Al haNisim" ends with the Haman's hanging, ending the danger to the Jewish people. The recounting of the Chanukah story goes a step further. We not only relate the miracle of the victory over the Greeks and the end of our persecution, but we add, "V'kav'u shmonas y'mei Chanukah eilu l'hodos u'l'hallel" -- we describe the establishment of the holiday of Chanukah as a reaction to the miracle. Why is the establishment of the holiday incorporated into the text of Al haNisim which thanks Hashem for the miracle of our salvation?
The Kedushas Levi asks why the miracle of Chanukah was established as a holiday and so many other victories over our enemies were not. The answer perhaps lies in a careful reading of the description of the holiday given in the gemara (Shabbos 21b). The gemara writes that in the year following the miracle "kav'um v'asaum yamim tovim b'hallel v'hoda'ah". If we were writing the text we would have probably said that the Yom Tov was established "l'hallel v'hoda'ah", similar to the text of tefilah where we say "l'hodos ul'hallel". Why does the gemara use the strange phrase "b'hallel" instead of describing the praise sung to Hashem, "l'hallel..."? The Sefas Emes answers that the day of Yom Tov does not in fact obligate us to sing praises to Hashem. Quite the reverse. It is our praises to Hashem which the miracle of the day elicited which led to the establishment of the day as a Yom Tov. It was through our hallel, "b'hallel", that the day attained its character. It was not the miraclulous victory over the Greeks which created Chanukah, as many other victories have come and gone without a Yom Tov being established. The Yom Tov of Chanukah was given to us because our hearts opened in response to this victory unlike any other.
I think this explains the different in the Al haNisim text as well. The establishment of Chanukah as a Yom Tov is not just a coda to the events of Chanukah, but is an essential aspect of the miracle itself. Unlike other victories which came and went, Chanukah's song of hallel made an everlasting impression on the character of the Jewish people and has left us with a Yom Tov l'doros.