Which method of decoration do you use/intend to use? Some brands are better made than others, and usually more expensive. There are different fabric weights. Ringspun cotton is considered to be better than not-ringspun cotton, and usually more expensive. There is 100% cotton, cotton/polyester blends, and 100% polyester, each with it's own purpose and preference by buyers. There are also fabrics designed to disperse sweat that are intended for people involved in sports - running, for example. There are different styles/cuts - fitted ladies, fitted unisex/men's, and boxy (square-shaped/classic) unisex/men's.
Choice is determined by decoration method; price/quality of garments you want to sell; gender, lifestyle, and perhaps age, of your intended customers.