It's always suggested to have your PI on the higher release
For PI, it is a prerequisite that no other system in the system landscape has a higher release than the PI system.
Reference : 1043047 - Compatibility of XI/PI in the system landscape
And as per the SAP upgrade dependency analyzer, independent upgrade i.e. ECC upgrade without PI upgrade is not advisable.
But it completely depends on the usage of ECC and PI. You might want to check with SAP before making any decision.