The higher the factory 'guaranteed' over-clock the higher the voltage they set at the factory to ensure that all of a line can run at the rated speed. When DDR3 first hit the scene the only chips that would over-clock a damn were E's…that could handle higher voltage and basically required it. Now that DDR3 is a mature technology we see 1.5v'ers at 'over-clocked' speeds....and 1.65 is only for really higher over-clocks. We are seeing the exact same cycle with DDR4 and I expect '4 to follow in '3s footsteps.
Unless I am building a true Epeen system for someone I use 1.5v DDR3 sticks when possible. Not because the extra voltage means anything, not because of extra heat (also minor)...rather simply because I know I can plunk in 1.5v sticks in ANY mobo and have them work....and I know that if the customer upgrades the BIOS later and 1.65v issues are still not fixed...I wont be getting a call saying the system wont turn on.
Personally I find performance differences to be rather small after 1600...and almost non-existant outside synth tests above 2133. Same with 9 vs 10 timings....too small to worry about.
Personally if I can find the room in my budget for 90 bucks for ram...I can find room for 130 and get 16GB. To me and my personal needs I find twice the ram has a bigger and noticeable impact in games and photoshop than pure speed.
BUT if all a system calls for is 8GB...get the cheaper GSkill you probably wont notice a performance dif between them all. BUT get two sticks not four....just in case you change your mind later on 16GB vs 8GB
Your results can/will/may vary.