if you are happy with 7.1 then stay with it.
Upgrading to 8.4, the latest, will give you all the latest bug/security fixes and Apple Music, if you are into that.
Hello folks! I'm using iPhone 5 and currently I'm using iOS 7.1 my question is that what if I go for iOS 8.1.1 ? what will be the positive and negative impact if I upgrade it to higher iOS versions ?