Which iMac should I choose

Definitely the new one in this case!
It's a new generation of CPUs and a whole lot faster (not like some former minor upgrades from 2,8 to 2,93 etc).

okay cool sounds good! I wanted to save up and get the newer model but i needed a little more convincing :p... I really wanted to like get the refurbished one because of the price but over this past year i really realized that you have to think about "the long run" when it comes to computers. esp at a price like this.

my other question is How is a 3.06Ghz duel core processor better than a 2.5Ghz Quad core?
 
i7 quad all the way. I got the 27" with it and couldn't be happier.