If one has enough money to buy Tzitzis or Tefillin and not both, which one should he buy? I heard Rabbi Akiva Aiger talks about it.
There is a dispute among the posekim about this.
Sheyarei Knesses Hagedolah (Orach Chaim 25:1) states that in such a case one should first buy a tallis, since the mitzvah of tzitzis is equivalent to all the other mitzvos. He also cites someone (I can't read the name of the sefer) who quotes Ritva, who gives another reason: tzitzis is a more frequent mitzvah, since they are worn on Shabbos and holidays, unlike tefillin.
However, everyone else disagrees, and say that tefillin take precedence. Mishnah Berurah (25:2) states that this is because tefillin is a personal obligation (chovas haguf), while tzitzis are technically an obligation on the garment (i.e., one is required to have tzitzis only if he's wearing a four-cornered garment, and theoretically one could go his entire lifetime without doing so - though it is true that the Gemara, Menachos 41a, criticizes one who does so).
Shaarei Teshuvah (25:1) cites "Ri in the name of Acharonim" who offer a compromise: if he can regularly borrow someone else's tefillin, then he should buy tzitzis first.