From my experience of Architecture, the computer labs at the department don't hold as many computers as you'd hope - usually resulting in being unable to get a machine, especially if deadlines are coming up. Whilst you can use any PC at the university, only the ones in the Archi department will have the needed software.
I used a PowerBook G4 15" towards the tail end of my degree, and a PowerMac G5 at home. This worked well, as I could work at university without waiting for a computer, and at home I had access to some decent power for late-night sessions. I used VectorWorks and SketchUp a lot, and if I absolutely had to use AutoCAD I used Virtual PC. Of course, these days you can use BootCamp which is a far better solution. The point is, Macs make a perfectly good machine for Architecture.
Some of my friends on the course also switched to Macs, and currently use MBPs (15"ers) now they're into the final stages. No complaints.
Plus, £200 you save not buying the 17" will happily go into the cost of making models.