I have to go with SplashID for keeping data. It has issues, but it keeps *everything*, and has been invaluable to me over the years both home and on travel. And it does (local, manual) sync with the iPhone, iPad, Blackberry, Palm Treo...app on everything platform.
On the flip side, I got a...
Time!achine backups to USB drives are notorious for bogging down a machine. I've noticed it for years. Maybe on current systems its now imperceptible, but on Core2 Duo machines everything's slower when Time Machine is running.
I use TimeMachine, SuperDuper, and Backblaze -- on both my macs.
TimeMachine has been specifically helpful to me in recovering deleted apps, lost emails, and reverting documents. For the minor cost of a hard drive, I recommend it without hesitation. I've not done a system recovery with it.
But...
Ignore the salesperson, get what you need. Look at your current hard drive use and go from there.
For a counter example:
I'm using 170 GB of a 500 GB drive on my MBP.
My wife uses 290 GB of a 640 GB drive on her iMac. That includes a 32 GB Windows partition, years of photos and all her...
Someone call me? ;)
At risk of asking the obvious, but do you need a laptop? My MacBook pro never leaves the desk now that I've got an iPad. (I'm too late...sounds like your wife needs a laptop to take to classes)
The Air is an amazing device; I always leave the Apple store lusting for one :)...