celebration or commemoration?

Why do we have a mitzvah of mishteh and seudah on Purim but not on Chanukah?  I think most people are familiar with the Levush's answer that on Purim, the threat to Jewish life was irrespective of religious commitment.  Even if someone had agreed to convert, Haman would not have spared them.  Since the threat was one of physical annihilation, the celebration is a physical one of eating and drinking.  On Chanukah, the Greeks did not want to kill us; they wanted to force us to abandon Judaism.  Had we given up Shabbos, milah, Torah, they would have left us alone.  Since the threat was to our spiritual lives, the celebration focusses on the spiritual alone, without the physical party.

The Taz does not like this answer.  גדול המחטיאו יותר מן ההורגו (Bamaidar Rabbah 21, quoted in Rashi).  Spiritual danger is far greater than physical danger.  Therefore, says the Taz, since on Chanukah we faced spiritual danger, we should make the bigger party then.  On Purim we faced the lesser threat of physical danger, and so therefore it should warrant a smaller celebration.  So why is it ונהפּוך הוא?  

Taz answers that instead of looking at the threat, we need to look at the salvation, the miracle.  Everybody was aware of what Haman planned to do and everybody celebrated his defeat and our lives being spared, so we in turn make a big party.  The miracle of the oil for menorah was different.  We got to do a mitzvah properly that we otherwise might have missed out on, and so we have a spiritual celebration, but that does not generate the same public excitement and merriment that having one's life spared does.

R' Shimshon Pincus (Tiferes Torash siman 19) explains the nekudas ha'machlokes here very nicely. According to the Taz, Chanukah and Purim are days of celebration, yamin tovim derabbanan.  The question is why the "shiur" of celebration is greater on one than the other, and so the Taz comes up with a sevara.  According to Levush, Chanukah and Purim are days of commemoration.  The days are inherently ymei chol, but there is a chovas ha'gavra to commemorate the nes.  גדול המחטיאו יותר מן ההורגו is not relevant because גדול המחטיאו יותר מן ההורג speaks to what the threat was, not what the nes was.  On Purim, the nes was physical salvation, so it gets commemorated with a physical party; on Chanukah, it was spiritual salvation, and so there is no physical celebration involved.