Why some RAM for the iMac 27 inch is 1.5V and some are 1.35V?
See my answer in this question on whether you can actually use the RAM and how to check how much RAM your computer is using at the moment.  It may be the case that you aren't even using what you currently have.  With regard to the RAM voltages, I'm not sure why they would be different apart from the sticks being slightly different designs.  You would be best off matching the sticks you buy to whatever voltage your current RAM is though.