If you are talking about i5 vs i7 on a 13" MBP then:
Performance on normal daily usage: you won't be able to tell the difference
Performance on CPU Intensive app: 15-20% faster (if I remember the benchmark results correctly). That means if i5 takes 10 sec the i7 will take around 8-8.5 secs
Battery life: worse on i7. About 15-20 mins
If you use your macbook on battery then the CPU will also be clocked down by OS X to save battery to an almost identical level between i5 and i7
You can search for more performance benchmark on the Internet!