Comfort is your number one priority with a shoulder holster. Since you'll be wearing it so close to your body, it's important that it's made out of a material that is comfortable and breathable. Neoprene and nylon are great choices. In addition, your holster should be ergonomically designed to fit. There's no point in wearing something that won't sit well against your body. You'll just avoid wearing it and it will be a waste of money. Look for a holster that is designed to fit well and that can be adjusted to your shoulder and waist size.