Big difference between desktop and mobile.
"The majority of previous-generation Core i7 and i5 desktop CPUs are quad-core processors, while mobile versions of both processors from previous generations are dual-core. ... The latest mobile Core i5 processors, meanwhile, mostly have four cores."
Comparing one i5 to another i5 can be hard to do unless you do some research. Within each of the Intel CPU families (i3, i5, i7,etc) are processors of varying clock speeds. The main thing the i7 has over i5 is hyperthreading and perhaps a bit more more cache memory. There are some high end i5's that will outperform lower end i7's on certain tasks.