The first type is an insert that slides into your existing shoes. These are little more than a sloped riser that can give you anywhere from half an inch to two inches in your appearance of height.
The second type of shoe lift is an actual customized shoe that has the riser built right in. This second type of shoe lift is able to give up to four inches in additional height because it is not constrained by the confines of the open area of the shoe.
In choosing between both types, you have to consider the pros against the cons. The list of pros for the insert shoe lift is that you can use it with just about any of your current shoes, its cheaper, and you are not limited to the styles that the makers of custom shoe lifts put out. The biggest con is that you are limited to about two inches or less of added height appearance. Custom shoes have the pro of the potential for added inches, but they are much more expensive and your selection of shoes is dependent on the styles that the manufacturer makes. That means that you can no longer buy and wear any conventional shoe that you see and like because you are now limited to wearing shoe lifts indefinitely because you have publicly shown the world that you are falsely two to four inches taller than you really are.
And therein lies the problem. Wearing these contraptions are akin to wearing a bad toupee. Instead of addressing the problem by focusing your efforts on increasing your height naturally through height increasing exercises, nutrition, and stretches, you are merely using a band aid solution.
You are now a slave to your shoes, particularly around people that don't know about your 'little secret.' You must always have your shoe lifts on at all times lest someone notice your lack of height from when they saw you with them on before.
This can no doubt lead to very precarious and uncomfortable situations when it comes time to become intimate with a potential partner. How would you explain how you were suddenly four inches shorter just because you took your shoes off?
If you are uncomfortable with your current height, shoe lifts are not the answer. Only true, natural height increase can truly make you taller.