Weight: If you want a super light frame titanium frames are generally very light.Material: You have the choice between metal or plastic. Metal tends to be lighter. Plastic frames tend to be more durable.Flexibility: This is a nice feature to have for kids or sports glasses.Color: Plastic frames tend to come in more color options, especially acetate plastic.Hypoallergenic: Stainless steel, titanium, and acetate frames are hypoallergenic.