Kiddushin 32a - Prioritizing mitzvos (kibud av)

The gemara says that we pasken like isi ben yehuda, that if one is faced with a mitzvah that cannot be done by others and the mitzvah of kibud av, he should choose the other mitzva. But if the mitzvah can be done by others then he should choose kibud av. Rashi adds that the type of mitzvah we are talking about is a mitzvah overes, that if not done now would be lost forever. This implies that a mitzvah that can be done later, even if it cannot be done by others would be pushed off until after the kibud av is done. This is all assuming that one has not yet begun to do the other mitzvah, but if he already began the other mitzvah, he would never stop for kibud av since we rely on the rule of osek b'mitzvah patur min hamitzvah as the Ran writes and is recorded in Rama [In my sefer, at the end of the chapter of osek b'mitzvah he'ara 18, i tried to suggest that the osek b'mitzvah rule would not apply to the mitzvah of kibud av. Meaning that if one was osek in kibud av and a mitzvah that could not be done by others and was overes came his way, he would have to stop doing kibud av and do the other mitzvah - see there].

The concept of a "mitzvah overes" by definition is a mitzvah that cannot be done by others and cannot be done later. If it can either be done by others such as pidyon shevuyim, or can be done later by me such as tefillin, then it would not qualify as overes.

The gemara is clear on daf 29b that a mitzvah overes will always come before a mitzvah that is not overes. It is for this reason that aliya l'regel comes before pidyon ha'ben. Even the Rabbonon who disagree is based on a pasuk that gives importance to pidyon haben, but by other mitzvos would agree to the concept. This is mefurash in Tosafos Ri Hazaken - 

והלכה כרבנן וכו' אבל בשאר מצות מודו ליה לרבי יהודה דטעמא קאמר, ושמעינן מיהא דבשאר מילי עביד מצוה עוברת ברישה לדברי הכל

Obviously, this rule would only help prioritize when one is faced with 2 mitzvos, one being overes and the other not overes, but when one is faced with 2 mitzvos each being overes, other criteria will have to be used to determine priority.

Based on this rule, the mitzvah of kibud av v'eim should rightfully be pushed off for any mitzvah overes. There is nothing special aboug kibud av in this case, because all other mitzvos would also be pushed off for a mitzvah overes. The chiddush in the gemara is not in the words of isi ben yehuda, rather in the word of elazar ben masya who implies that you would always forgo kibud av for the sake of another mitzvah since the father is also obligated in other mitzvos (at least m'din arvus) he has not ability to use his power to prevent the fulfillment of that mitzvah. Meaning, in the presence of another mitzvah, kibud av doesn't exist. But Isi ben yehuda says that kibud av follows the same rule as any other mitzvah - since it is not considered a mitzvah overes, one must forgo kibud av for a mitzvah overes, but not for a mitzvah that can be done by others or can be done later.

By Talmud Torah we find a similar distinction that for a mitzvah that cannot be done by others one must stop learning. But the chiddush by Talmud Torah that the gemara says in mo'ed kattan is that even though talmud torah is also a mitzvah overes, since every minute is an independent mitzvah, you would still push off talmud torah for another mitzvah. This is unique to Talmud Torah for 2 reasons: 1. Talmud Torah itself is a mitzvah overes. 2. the rule of osek b'mitzvah allows one to continue even at the expense of a mitzvah overes, yet by talmud torah we insist that he must stop learning.

For more discussion of prioritizing mitzvos and criteria that should be used, see R' Akiva Eiger Teshuvos (siman 9) and also printed in shulchan aruch beginning of hilchos mezuza.