Hi there drkraj, the i7 processor will definitely give you much more processing power, if you are required to run demanding software such as CAD or photoshop, go ahead with the i7. However, if you are more concern about battery life and you doesn't need to run any power demanding software I would suggest you to go for the C2D processor.
i want to buy lenova think pad , one is having Core 2 due and other is with Core™ i7-720QM processor. Which is the better one to choose.
As the other users stated, the i7 processor will be more powerful. However, such a powerful processor may not be necessary for what you will be using your laptop for. For a great explanation on processors and processing speed, check out this easy-to-understand guide from PC Scout: http://www.microsoft.com/Windows/pc-scout/laptop-speed-....