If your car has a blocking fuel tank cap, then there is little or no need for a blocking fuel cap. This is because a dedicated thief would be needed to pass the fuel tank cap and if someone is like that, then it is likely that a closed fuel cap will not stop them. If you have one, you don't have to worry as long as you lock your car doors. As you are likely to do this anyway in an area where your fuel could be stolen, you do not have to change anything to benefit from these.
If you live in an area with high rates of fuel theft or even just theft in general, then you are likely to benefit from a blocking gas tank cap. Even if you never prevent someone from stealing your gas, it is worth making a small investment to be calm. Also, if you prevent the theft of a single tank, it is likely that it was paid only based on what you paid for it.
Anyway, if you are replacing your fuel cap, you are going to spend almost as much as you would on a fuel tank cap. There are many reasons why you may have to replace your gas cap, but the most common is a defective seal. As the plug wears out, the seal will also begin to wear out. Once this seal is no longer maintained, you can start getting a random check engine light that is difficult to diagnose but is often fixed simply by replacing the fuel cap.
Blocking fuel caps is an excellent way to prevent fuel theft. While there are ways to avoid it, they will prevent most opportunistic thieves from stealing, although they may not allow a dedicated thief to be arrested. Someone who is willing to make a hole in the gas tank will not be deterred by having to deal with a blocking fuel cap.