Speed of wave propagation is determined by two properties of the medium, the elasticity and its density. Elasticity and stiffness are related, but different concepts. Elasticity refers to the ability of a medium to resist deformation while stiffness refers to the ability of a material to resist force. Generally, we think about a material that is very elastic as lacking stiffness and vice versa (suggesting an inverse relationship), so you can use your understanding to help you understand the medium and predict how it might affect the speed of wave propagation - but for the purposes of the equation, it is elasticity that is considered the main factor to influence wave speed.