If you want the screen real estate, go for the 17"; for portability, choose the 15"; in terms of performance, the i5 and i7 aren't that different; just a few numbers. When I had a 17" it was too big for most desks, never fit on an airplane (in coach at least). 15" is a good size and now with the higher resolution displays it's a great choice.
I hope you know the OP is asking if the choice should be a 15" i7 or a 17" i5
Go Big or Go Home.
Get the i7. You cant just upgrade it later like you can Ram or a Hard Drive.
Also with the i7, you get 512MB of Video Memory as oppose to 256MB.
Think of it as future-proofing your purchase.
I hope you know the OP is asking if the choice should be a 15" i7 or a 17" i5