If both sex and somatic cells have sex chromosomes, how are they different?
A sex cell is formed as a result of meiotic cell division--a reduction division that produces a haploid sex cell from the parent's diploid body cell inside a gonad. Because it is haploid, it will contain a single sex chromosome, either an X or a Y chromosome.
A somatic cell is a body cell that is diploid and produced as a result of the fertilization of an egg by a sperm cell leading to a diploid zygote or fertilized egg. From this cell, mitotic divisions occur resulting in a diploid organism. All of the somatic cells within that organism are diploid.
During fertilization, each gamete or sex cell contains one sex chromosome and it is the fusion of the nuclei of these gametes that results in the production of a zygote with its chromosomal sex established--either XX or XY.
So, to reiterate the difference--a sex cell contains only one sex chromosome because it is haploid and a somatic cell contains two sex chromosomes because it is diploid.