You mean it's ``just a UNIX machine with...'', no?
I like Macs but I would be concerned about they're being so ``closed''. Need more onboard disk space? You need to buy another Mac. (I'm talking about the ``Airs''; not sure about the bigger, more expensive models.) Or another disk to toss into an external disk dock. (I love those things.) At least with a ``Lintel'' laptop, upgrading the internal disk is a simple matter. I'd opt for the big internal drive to avoid having to lug around a lot of external gadgets but that's just me. (Of course, I'd want at least one extra disk as big as the internal for backups onto the external dock waiting back in the dorm.)
If you're worried about being able to haul the laptop into a repair shop, then buy a Windows-based system known to work well with Linux (get the one with the smallest hard disk they sell), immediately pull out the hard disk, install a new one, install the Linux distribution (Scientific sounds appropriate for a physics major), and go forth and be productive. If any hardware problems arise, pull out the Linux HD, re-install the Windows HD, and have the repair guys work on it using an OS they're likely more accustomed to using. And no support hassles about having replaced Windows.