What is the difference between Mitzvah Goreres Mitzvah and Schar Mitzvah Mitzvah?

Question

The Mishnah in Avos (4:2) says:

בן עזאי אומר הוי רץ למצוה קלה ובורח מן העבירה שמצוה גוררת מצוה ועברה גוררת עברה ששכר מצוה מצוה ושכר עברה עברה

Ben Azai says, “Run toward a light mitzvah and flee from an aveirah, for a mitzvah leads to a mitzvah, and an aveirah leads to an aveirah; for the reward of a mitzvah is a mitzvah, and the reward of an aveirah is an aveirah.

The Mishnah seems awfully repetitive. Why does Ben Azai use both the phrasing of “a mitzvah leads to a mitzvah” and “the reward of a mitzvah is a mitzvah” (and likewise by aveiros)? Don’t they mean the same thing?

Answer 1

See Rabbeinu Yonah's explanation.

In summary, the first part "for a mitzvah leads to a mitzvah" is a further explanation to Ben Azzai's principle mentioned in Avot 2:1. Here, Ben Azai points out the concept of developing a habit. When one performs an easy mitzvah, he becomes closer to G-d and develops his spirit to want to do another one of the same level, or perhaps, a mitzvah that requires a bit more effort. Eventually, as he does more mitzvot, he develops a natural habit of continuously doing more and more mitzvot to the point where he does it more quickly until he has performed all the mitzvot.

The 2nd part about the reward is pointing out that the concept of good an evil is not in the hands of man, but G-d only gave man the choice of doing good or evil.

the fruit of [doing] a commandment is [doing another] commandment; and he eats its fruits in this world, which is that it helps him to do other commandments. And it comes out that the principle expands and it exists in the world to come. And this is what is stated (Isaiah 3:10), "Hail the just man, for he is good; they shall eat the fruit of their works."

Answer 2

The Bartenura on this Mishna explains as follows:

שֶׁמִּצְוָה גּוֹרֶרֶת מִצְוָה. כָּךְ מִנְהָגוֹ שֶׁל עוֹלָם, הָעוֹשֶׂה מִצְוָה אַחַת נוֹחַ לוֹ לַעֲשׂוֹת אֲחֵרוֹת, וְהַמַתְחִיל בַּעֲבֵרוֹת קָשֶׁה לִפְרֹשׁ מֵהֶם.

וְעוֹד, שֶׁשְּׂכַר מִצְוָה מִצְוָה. שֶׁמִּן הַשָּׁמַיִם מְסַיְּעִין וּמַזְמִינִים בְּיַד מִי שֶׁעָשָׂה מִצְוָה אַחַת שֶׁיַּעֲשֶׂה אַחֶרֶת כְּדֵי לָתֵת לוֹ שָׂכָר עַל שְׁתֵּיהֶן. וְכֵן שְׂכַר עֲבֵרָה וְכוּ'.

פֵּרוּשׁ אַחֵר, שֶׁשְּׂכַר מִצְוָה מִצְוָה, שֶׁכָּל מַה שֶּׁאָדָם מִשְׂתַּכֵּר וּמִתְעַנֵּג בַּעֲשִׂיַּת הַמִּצְוָה נֶחְשַׁב לוֹ לְמִצְוָה בִּפְנֵי עַצְמָהּ, וְנוֹטֵל שָׂכָר עַל הַמִּצְוָה שֶׁעָשָׂה וְעַל הָעֹנֶג וְהַהֲנָאָה שֶׁנֶּהֱנָה בַּעֲשִׂיָּתָה:

"Since one commandment leads to another commandment": That is the way of the world, that one who does one commandment finds it easier to do others; and one who starts to do sins will find it difficult to separate from them.

And also, "since the reward for a commandment is [another] commandment;" [meaning it] is from the Heavens that the one who does one commandment is helped and another commandment is placed in front of him to [also] do, in order to give him the reward for both of them. And so too, "the reward of a sin, etc." 

Another explanation: "since the reward for a commandment is [another] commandment;" Since all that a person is rewarded and derives enjoyment while doing a commandment is itself considered a commandment for him. And [so] he gets reward for the commandment that he did and for the enjoyment and benefit that he experienced in doing it.