I had my heart set on the 17" for a LONG time and up until yesterday thats what I thought i was going to buy. Until I talked to a few people that have MBPs. I was told that the 17" is just way too big, it loses a lot of portability, my friend said that if you are thinking about taking it on a plane... the 17" will not fit on the seat back, also that when using it on your lap its just hard because it is so large. Also not as many companies make cases and sleeves for the 17", of course you can find them but its not as easy, and you don't get as much selection.
Also on a technical side... remember the 15" screens are LED backlit therefore increasing battery life and they are more environmentally friendly.
On the other hand, the 17" are not LED backlit but they have an option to get an HD screen.
Really I would say go to your local Apple store and look at them both and see what you prefer.
I know that after talking to my friends I will be getting the 15" not the 17" as I had originally planned.