Hi
cihan.kizilarmut,
Welcome to the forums

That is a difficult one to answer and will be different for each user.
First you need to read each of the following News posts and decide is there any feature mentioned that you need to resolve a particular problem or that you think would be cool to have.
https://peppermintos.com/2016/11/peppermint-7-respin-released/ Peppermint 7 Respin
https://peppermintos.com/2017/05/peppermint-8-released/ Peppermint 8
https://peppermintos.com/2017/12/peppermint-8-respin-released/ Peppermint 8 Respin
Each new version brought new things to the table and you will have to weight up do you really need any of those new things.
For me the biggest question is - is everything running perfectly OK on the P7 install - if the answer is yes then stay put. Both versions are running off the 16.04 LTS Base and will be supported until 2021.
If the answer is no - what is the problem ?
have you sought a solution on the Forum that has not been resolved?
have you read something somewhere that suggests a new kernel might be the answer?
There are simply too many factors to answer this with a simple Yes/No type statement.
Other members of the Forum will come along with their opinions/ideas/suggestions and that is one of the strengths of this community, a diversity of answers to help you decide whether to upgrade or not.
Regards Zeb...