Core i7 is the leader in high end (AMD no way near) processor market. Core i5 is Intel's new mid range processor.
* Turbo Mode
Core i7 can over clock itself when some cores are not being used (up to 300 MHz boost).
* DDR3 Memory
Core i7 requires RAM in set of three.
Core i5 requires RAM in set of two.
Core i7 has 4 cores but can simulate to 8.
Core i7 costs about $100 more than Core i5.
Is Core i7 worth the extra $100? Yes and No. Like with any other high end product we can't justify the price. For 99.99% of the people even the budget range of Core 2 Duo is more than enough and the difference is not noticeable.
If you want the best: Core i7
Value for money: Core i5