intel quad core i7 2.9hz vs imac 27 3.1 ghz i5

My old iMac packed up recently, overworked!
As I sadly do not have the money to buy the latest quad core 3,5 gig retina I have to look second hand.

I don't do much video or 3D editing but have often very large stacked or stitched RAW files. Both are done in Adobe programs. While those programs running I don't really do much CPU intense other tasks

A short explanation which model is more suitable and why would be much appreciated.