There are three different types of writing customs for the parchments inside tefillin: Beit Yosef, Arizal, and Vellish. Beit Yosef is the same script that Torahs are written in. Arizal script is only slightly different from Beit Yosef (seven letters differ), and is the preferred script of the Hassidic community. Vellish script is bolder, and is normally used by those of Sephardic descent.
According to the Arizal himself, each of the various customs is authentic, and all are imbued with deep meanings.
The quality of the writing on the parchments varies dramatically and is one of the main factors in determining the price of the tefillin. Because the writing on tefillin parchments is considerably smaller than the writing on a mezuzah or a Sefer Torah and writing small is hard to do well, writing a set of tefillin parchments takes much time and effort, making good tefillin parchments expensive.
Cheap tefillin are written more quickly. Often, they have been written so quickly that not only are the letters far from elegant, but they are barely kosher.
Moreover, low-end tefillin are written on parchment that has been treated to make it easier to write on, but this treatment accelerates the decay of the letters. So even if the letters do come out kosher, they are likely to decay ten years down the road.