The ECO are designed to use less power (voltage), which is why they are 1.35V. If you are overclocking the ECO gives you more wiggly room with the voltage settings. With the lower voltage requirements, it gives you more room to adjust voltage to help with stability in overclocking or running tighter timings. Running the Ripjaws at 1.65V, you don't have room to adjust the voltage up to make an overclock stable.
either / or should work. They are rated at 1600. Currently have the Ripjaw 1.6V CL 7-7-7 and My system (I5-750) runs stable @ 3.8 GHz (bcklk = 200 and cpu mult @19x). Note I ratched it down to 3.2 simply because I do not need the 3.8 - BUT it's there if I do. My memory is set to use XMP profile 1.
Of the two I'd probably go with the 1.35 V. On that not, I'm not sure about their power disipation, so would not push above 1.5 V.