I've had both the low end 15" Unibody and now I own the standard config 17" unibody. I had 3 15" models and all of them had poor screens, so with the release of the new 17" ( my first 15" was in November ), I decided to put another 3 weeks pay towards a better unit.
I've had it about 6 weeks now and I would definitely say go for the 17".
Reasons :
Obviously the screen - I do a lot of Photography and the colors are more vivid, plus the extra 2" makes a real difference for me ( ArchiCAD is awesome on this screen, more room ' to build' )
Battery - I took my Macbook Pro to college for the first time today and I was able to get a full days work out of one charge, eliminating the need to take the power brick etc.
Speed - It seems a little faster, but not a whole deal so don't get it for the speed as it's won't justify the price difference.
But luckily I got it for the ' main' upgrades such as the screen and battery
I'm a college student ( going to Uni in Sept. to study Architecture ) and this has been the best purchase I've ever made, not only because of how stylish and ' clean' it is, but because OS X is simply the best OS for me, and I'm never going back !
NStocks