In the circumstances you have described, I would consider the cost of the gel polish as Supplies Expense. In a service business such as yours, supplies would be anything you purchase that contributes to the completion of the service. If the occasional bottle of polish is sold to a customer, record the sale, but there is no corresponding CGS, as you have already booked the expense.
If you were selling bottles of polish directly to retail customers, we would look at the transactions differently. Then, the polish would be purchased into inventory. It would be removed to CGS when actually sold, with the year-end product remaining in stock carried as Inventory.
There is no choice to be made as to which is "better." It is simply a reflection of the kind of business you are conducting.