You will notice a performance difference for some tasks. The biggest difference is that some i7 processors are quad core while i5's are all duo core. When comparing a duo core i7 to a duo core i5, it will have a slightly higher clock rate. When comparing a quad core i7 to a duo core i5, the two extra cores will make a considerable difference for heavily threaded applications. You will be able to do video encoding much faster. Some games like sc2 will disable hyperthreading in the processor, and will benefit from quad core as well.
If you can provide the model of i5 and i7 you are comparing, and also provide a list of things you do on your laptop, we can help you decide.