That's a wide-open question. There are reasons you might want an i5, there are reasons you might want an i7.
About the only thing I can say so far is that you will almost certainly want >4GB of RAM, and that means running a 64-bit operating system. I'd suggest 8GB and Windows 7 64 bit. I'd also suggest getting one with USB3, because it's so much faster than USB2, and much more convenient than eSATA.
Look carefully at the specs of the laptops you consider. Bear in mind that some (all?) laptop i5 CPUs are two core, not four, so they are not comparable to the desktop CPUs.