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.
I'd go for the cheapest Core I7 quad core with the 6mb cache. I priced a sager and the upgrade cost to this CPU was around $150 over a core I3 or I5. Not bad for a quad core with 6mb. It's the only way your going to get desktop performance with a notebook. Also with a good 3 year warranty you your keeping it for at least that long without any upgrades. Might as well have something that will take you the distance then still have some value.