Could not make it to shule last Shabbos. On Rosh Chodesh an abridged Hallel is recited on Chanukah the full Hallel is recited. Last Shabbos both occurred together which version is recited and why?
Not sure if the usual (Rosh Chodesh) trumps.
We recite the full version. We are obligated to say the full Hallel on a Yom Tov etc., and days we are forbidden to do melacha. Nevertheless the minhag is that we also say it on Rosh Chodesh, however in order to differentiate between the obligational times and when it is merely due to the minhag, we only say a partial Hallel on Rosh Chodesh. Chanukah tough is different. When the Rabbis instituted the Holiday of Chanukah, it was with an obligation to say Hallel, therefore reciting Hallel on Chanukah is not a mere minhag but an obligation. The reason we say a full Hallel on each day of Chanukah (in contrast to Pesach, when we only say a full Halle on the first day(s) of Yom Tov, is because each day was considered a separate miracle, and because each day we read a different days korbanos. Since Chanukah require a full Hallel, we say the full version, on all days of Chanukah, even on Rosh Chodesh.
Mishna Berura 422-12, 683-1, Tractate Shabbos 21b, Rashi ibid V’osim Yom Tov. Bais Yosef O:CH 693.