The key difference between the two stamps is that with the self inking stamp what you need to do is to hit an ink pad and flip the cover back down. These stamps work great for leaving more 7000 impressions and they can be filled with ink again.
On the other hand, there are pre inked stamps which contain ink inside of them directly on to the words. A pre inked stamp is much different from a self inking stamp. The major advantage of these stamps is that they can last up to more than 50,000 impressions. Ink can also be filled in them.
What is the difference between polymer and natural rubber?
All of the stamps come with built in polymer until and unless you decide to get a natural rubber upgrade for your stamp. The polymer is made out of plastic which is great for many applications. In industrial type solvents or inks a natural rubber stamp is recommended to be used as a polymer will be easily rotten away.
Every stamp is perfect but that is according to some set of requirements and it works great for the precise environment that the stamp has been designed for. Not every stamp however can work on every kind of paper.