http://revach.net/article.php?id=4312

Section: Parshas HaShavua   Category: Quick Vort
Parshas Bo: Rav Meir Shapiro - Many Months Of Miracles
Rav Meir Shapiro says that although the words "Zeh" and "Hu" both mean basically the same thing, "Zeh" which means "this" is referring to something that is here before us now while "Hu" refers to something that is not here and cannot be seen.

This fits in well with the pasuk referring to Nisan as the Rosh Chodashim.  Nisan was chosen as the month for Yetzias Mitzrayim because it is a month for Geula.  The final Geula, says the gemara (Rosh HaShana 11a) will also be in Nisan.  Nisan is a month of miracles starting with Yetzias Mitzrayim and repeating itself every year.  Furthermore, says Rav Meir Shapiro, all the Nissim that take place the entire year have their roots in the month of Nisan, regardless of when they actually occur.

Regarding the Nisim of Nisan, the Torah tells us, "Hachodesh HaZeh Lachem", this month is a special time where you will see the Nisim, which is why it says "Zeh".  "Rishon Hu Lachem L'Chodshei HaShana", it is the month from which all the months take their miracles from.  Since the power of Nisan is not obvious in the miracles of the other month, here the Torah uses the word, "Hu".