You should price systems going either way and see what the difference is. When I was contemplating some new CAD workstations about 5 months ago I compared prices and an i7 machine would cost me about $300 more than an i5, for very little positive benefit. At the time the CPU was $90 more, the ram was about $100 more, and the motherboards I selected were about $100 more. At this time the motherboard prices are closer, the ram pricing will depend on what configuration you need, and chip prices are still about the same.
As I understand it most i7 motherboards use triple channel memory so you can have ram in multiples of 3GB whereas i5 boards use ram in multiples of 2GB. If 3GB is not enough for you but 4GB is, then you pay a penalty with the i7.
If you are a Junior and you want to use the machine for 5 years, that might be pushing it. Most Engineering schools don't get you into anything intense for the first 1 or 2 years anyway, in which case you could get by with just an ordinary laptop or desktop until then. When the time comes when you need the supercomputer, you can build it then.