The answer to your question depends on how you define "durable" in the context of a particular cotton garment.
Mercerization increases the tensile strength of cotton fibers by slightly altering their chemical structure. It also increases their resistance to mold, such as mildew. On the other hand, mercerized cotton is more attractive to dust and other fine, particulate matter.
On the balance, these qualities may or may not noticeably affect the durability of your cotton items, depending upon what you put them through and how you maintain them. Generally speaking, wear and care will make a much more significant difference in the long run.