Lower is better on both timings and voltages.
Although it basically comes down to , you can pick any two out of the three -- low voltage, low timings, or low price -- but it's hard to find all three together.
Also, if you aren't comfortable messing around with timings and settings in the BIOS, I'd just get 1.5V RAM and be done with it. 1.5V is the one you can most often just plug into a DDR3 motherboard and it will work right out of the box. It's not hard to change those settings, mind you, but that's one way to make it slightly easier on yourself.