Can you suggest a good model in Dell Laptop.I am confused  

i3 and core2duo are the same in that they are both dual core processors the i3 is the lowest member of the "i" core family at Intel so the i3 should be a better proc than the core2 duo so off of just proc you would be better off using the i3 because it is that cutting edge and I am asuming you are using the resource hog Windows so yeah I would say really you would want the i3 but you could get the core2duo at the same speed and it would do about the same as far as preformance if you got the same clock speed as the i3. So which ones are faster in clock speed. They are both dual cores and ones just newer than the other.

In some cases 'newer' is better, but I've worked with the cor2duo for quite some time and find it to be quite reliable. And, I've got a screen reader constantly running in the background which eats up valuable resources. But, without it I wouldn't be able to use the computer so that's why I use it.

there's nothing a best or worst
its only the price u can pay for the features
diff companies offer diff price due to reliability , service , brand name etc
so check out on the dell website
aLSO CHECK the ebay for current prices u can sketch a rough figure about the price graph