When buying tefillin you should be aware that there are different levels of quality. Although the gasos is considered to be the best and the most durable, you should buy the set you believe is most suitable for you.
Peshutim are the most simple. They are made of several pieces of parchment that are glued together, and are not considered to be of high quality.
Peshutim Mehudarim are made of one piece of parchment that is cut in a way that allows it to be folded and glued to create a solid box. They are light weight and have thin sides. They are vulnerable and in most cases will not last over time. They are less preferred by authorities because of the glue.
Dakos are made from thin skinned animals (such as a goat). The single piece of hide is stretched over and glued to an upper frame of thicker hide in the shape of a square and a lower frame which serves as the base. Both the upper and lower sections are considered to be made from one solid piece of hide. They are quite durable but still not as preferred by authorities as the gasos are.
Gasos are made of one solid piece of hide from a thick skinned animal (such as a bull). The process of creating them is long and requires great expertise. They are of high quality and will last over many years. Gasos are the tefillin most preferred by authorities.