Depends on how anal you are. I've interchanged between 15s and 17s the last couple years. I notice absolutely no difference in weight when carrying them around in my backpack. 17" is definitely bigger, but I only notice it when the screen is open and I have a lot more pixels staring me in the face. This notion that the 17" is this beast compared to the 15" is stupid. It's 1 lb heavier and it's NOT that much bigger, physically.
MovieCutter is right, this is not so big of a difference, and in the end my dad's Macbook with his charger, cables and other accessories gets to be about the same weight.
Also, this really depends what you want to do and what the 15" would be. If you are going to buy a 15" with similar specs to the 17", then you have to think about the screen.
The 17" which I love, has its downsides, specifically if you don't need the screen size and you're really mobile, then the 15" is probably a better option. Also, the heat generated by the 17" is a little bit more. But most importantly the 17" is kind of similar to a drug, if you don't need it, be careful, it's hard to leave, I love the smaller Macbook, but I couldn't use that for a day, So, if you'are a basic student switcher, and were like me you just wanted the ultimate top end, I'd say get the 15" and then when it comes time to buy again, see if you really want that 17", most people that have the 15" don't complain about screen size...plus if you are dropping 3 grand, you should be able to afford another computer later.
