How does closing the lid affect noise, since the exhaust fans are at the rear?
Only YOU can answer which is better for you. I personally could not live with the massive size and ridiculously high DPI of the 17." When I need more pixels and better color accuracy, I connect my 13" to my TV. For everything else, the WXGA res is sufficient.
If you do not use files that require larger than WXGA res., then you are paying a heck of a premium for extra pixels that you don't need. The price differences between the 13" and 15" are enormous. With $500, you could get a very nice 24" LCD.
Then again, if you don't plan on connecting your MBP to an external display often, then the 15" is probably the best compromise of size and portability, as well as price. I simply could not sleep knowing that my 17" MBP cost so much more than quad-core PC laptops.