Both is great to have. Stackmat is great for on the go solves, and casual sessions. it's also great for getting ready for a competition. if you want to improve or just track all of your solves from single, mo3, ao5, ao12, ao100, global, etc then you absolutely need cs timer. You can even plug your stackmat into your computer and connect it to cs timer! If i could have one over another i would just do cs timer, it's free, and it's what everyone uses for tracking times and improvement.