I have the iPhone 4S should I get the 5?

What is better about the 5?

The five is slightly larger, but the new one is gonna come out soon so just wait for that

At this point wait for the 5S which is due in less than a month.
If you do want the 5, compare the 2 & only if the 5's features are compelling enough then by all means…

Its lighter, thinner, faster, and the siri is better.

Wait until the new iphone is announced which is only a few weeks away now, especially since you have 4S as that is already a really good phone.

Wait for iPhone 5S, that is worthy for you to own.