For $200 you get:
-A few extra DPI (it's really not an extreme difference from the 15)
-An extra pound
-2" additional diagonal space (both visible and "handrest" space); makes the device more of a desktop replacement and a little less of a "lap thing"
-An ExpressCard slot (but no SD card slot)
-A battery with 15 more Wh (not that you'll ever notice the difference)
-The ability to watch and edit 1080p videos without downscaling, which is the real benefit. Remember: at 16x9, 1080p video means 1920x1080. At 16x10, a 1680x1050p display is 15% smaller.
Ask me it's a no brainer. You're better off with the bigger machine unless you can't carry 6.6 lbs in one hand. But then I love high dpi displays. My dream was a display with the same pitch as a photograph, and in 18 days it comes true. For those who think simply more pixels == more eyestrain, or that 720p is just is good, get the 15" and get on with your life.