Z series is better. Generally they have better VRMs, better power phase, better BIOS UEFI environment and better overall stability. That being said, if the board you are using, not just you, but whoever, is capable of being overclocked, then it just depends on what board and bios version it has, what cpu you're planning to OC and how much you want to overclock.
Is this a board you have or are you looking to upgrade to something?
As far as the RAM goes, aside from overclocking by configuring the settings to match the modules specs, if they exceed what the bios wants to default them to, I don't recommend overclocking the memory. The gain is minimal and the potential for damage or instability is increased. If your board, for example, says it supports 1333, 1600, 1866 (OC), 2133 (OC), etc., and you want to run 1866mhz modules which ARE supported by the board but require an "Overclock", that's fine. It's not "really" an overclock, not like trying to run 1600mhz RAM at 1866mhz or higher settings.
Making changes to the memory configuration to match the modules "correct" settings is fine, but trying to increase the modules performance beyond it's profile is not worth the effort and trouble it invites, in my opinion, and unless you have to do it in order to make changes elsewhere in the configuration more stable.