Bump!
A little late to reply to this, but if you haven't made a decision yet, I think I can offer some advice (Engineering Master's student).
The first bit of advice is more related to engineering rather than the computers. Engineering is a difficult program and perhaps the one in which sees the largest drop between the number that start out, and the number that actually graduate. It's not to say that you aren't capable or committed, it's just that it isn't for everyone, especially planning for a Master's degree at this point. Also, I've seen many people come to school with a maxed out top of the line computer which they justify because they "need it for school". A lot of them end up wasting time playing games because they have a computer capable of doing it.
As others have said here, expecting 6 years out of a computer is pushing it, especially when your computing needs are going to ramp up the further you go.
Starting out, you're going to be doing a lot of pencil and paper work, getting the fundamentals down. This will pretty much be your first 2 years. You won't need much more than Word and Excel for this. Anything else is going to be expensive site license software that you'll have to work on in the labs. Therefore, my suggestion is to go with the MacBook (especially if they update in June/July), save some money and put it towards a good calculator (TI-83, TI-89, or HP), and maybe an extra monitor for dual displays.
Once you get to third/fourth year, you'll probably start doing more complex stuff that requires heavier computing. At the end of your third year, you should have a better idea if you're going to stick around for a Master's or not, and whether or not you need the extra computing power.