A big factor in how a footbag will play is the fill type. Footbags filled with sand are generally softer and play with a fair amount of pop and bounce. Footbags with metal fills play a bit heavier. Plastic fills offer a good mix of play style in-between sand and metal. Once you've discovered your preferred fill type, it's important to factor in the number of panels and the fill level.
Sand filled footbags offer the lowest fill profile, allowing for a softer, more finesse style of gameplay. Sand types generally range from fine to larger grains.
Plastic filled footbags offer a great pop and bounce to gameplay. Generally the plastic pellets are a few millimeters in diameter.
Metal filled footbags offer the heaviest fill and can provide a more deliberate style of gameplay. Each metal pellet is generally a few millimeters in diameter.