Boostrix looks after 3 diseases . They are Diphtheria and Whooping cough in addition to Tetanus. If you want it for yourself then Inj Tetanus is good enough.
If you want it for your wife who is
pregnant then Boostrix is better because it will lead to formation of antibodies in the pregnant woman and those antibodies will be passed on to the unborn baby.
If you want it for your child, they have a special injection for the three diseases.
Now second part of your question,
When you want to buy a car, do you buy a car or you buy a bus, considering that bus will also have 4 seats ( bus may have many more seats but you are not bothered).