Choosing the right system for you will depend on the reveals in the window opening and the desired impact you are hoping for and budget.
Traditionally, all sliding sash windows were operated by weight and pulley systems. A box is built into each side of the window, which houses the weights. This is then attached to a piece of sash cord, which travels over a pulley in the top corner of the window and attaches to the sash that will be moving.The weight is chosen to closely match the weight of the sash, so that a perfect counterbalance is achieved and movement of the otherwise heavy window is effortless.
Spiral Balances function using similar principles of counterbalancing, but achieve it through the use of two springs inside a PVC tube. Spiral balances are visible on the inside of the window. They are typically the more economical option. Spiral balances systems are well suited to tight building/reveal profiles where space is restricted without reducing light.