A shoe insert goes inside the shoe and doesn't change the appearance of the shoe. A shoe lift is applied to the outside of your shoe, usually to the heel. It has several disadvantages.
Each pair of shoes must be altered to benefit from the lift. One shoe is always going to look slightly different from the other shoe. It's not easy to add a heel lift to a tennis or sport shoe. The heel lift also puts the ankle into a slightly flexed position. This can cause shortening of the Achilles' tendon and lead to other problems.
The advantage of shoe inserts is that they fit into most shoes including dress shoes and tennis or sports shoes. They don't work well in sandals.
With either choice research shows that the major effect starts soon after you begin using one or the other. The effect is maintained as long as you wear them. Both types must be replaced when they wear down or wear out.