Hi and welcome to computer forum!
The 2 different RAM voltages you are comparing are DDR (2.2V) AND DDR2 (1.8V). DDR2 is a new type of RAM which uses a lower voltage. Which one you choose depends on what type of RAM your motherboard supports. This information can be retrieved from your motherboard manual.
If you have any other questions, myself or any other friendly member of computer forum would be delighted to help you.