Why does the Torah sometimes mention Moshe before Aharon (see for example, Pasuk 13 and Pasuk 7), and sometimes Aharon before Moshe (such as in this Pasuk)?
Rashi: To teach us that Moshe and Aharon were on a par with one another (in righteousness).
Rashbam, Hadar Zekenim (verse 20): Whenever the Torah refers to the Shelichus, it places Moshe first (since he was the chief Shali'ach, whereas wherever it refers to the Yichus, it places Aharon first (since he was older than Moshe).
Why does the Torah see fit to insert "Hu Aharon u'Moshe ... "?
Rashi: The Torah is merely clarifying the identity of the current two leaders.
Seforno: It is pointing out that Moshe and Aharon were the most prestigious of their family, and were worthy of taking Yisrael out of Egypt, since their words would be accepted by all of Yisrael.