Well, I am a sucker for processing speed and screen real estate, so I'd say if you were planning on using this as a gaming rig in your spare time, go with the 15".
Caveats:
- I know nothing about the i5 processors.
- I have last year's 15" which indeed weighs a crap-ton
- I don't know the difference between those two nvidia cards, so I'm assuming bigger numbers are better, but given nvidia's history with laptop video cards, I wouldn't expect the stars out of these things.
- I do periodically make decisions based on how pretty something looks.
- Any of those machines will be fine for your day to day computing.
- A device weighing a metric crap-ton is just fine if you don't bring it to school every day. (I don't anymore)
As fopr the magic mouse, if you don't like it, you can try selling to one of your eager classmates or on ebay. I'd keep the external, though, as it's always handy to have an extra terabyte around, unless you've got plenty already... And let's be serious here. No one ever has enough.