Should I update my iPhone 4S to the newest iOS update?

I still have this really old update, and I can't decide if I want to update it or not.

It is a choice that you have to make. Do consider all that is mentioned in the link below. It is bit old new but tells you the basics of installing iOS 8 on iPhone 4S

http://mashable.com/2014/09/25/iphone-4s-ios8/

I think latest updates slows down the phone.
But if you're okay with phone getting slow, then go for it.