Having just got my 15 inch i7 MBP I'm quite surprised at the weight of it compared to the 13 inch MBP which I owned late last year. Sure the 13inch model is a lot easier to carry around due to its size but I don't really think theres much in the weight to be fair. To counteract the extra weight and size, I got a decent targus bag which has a padded shoulder strap and tonnes of storage room and when I put the laptop and some books into it, its not much heavier than sticking my MBP 13 inch a sleeve and sticking that into my uni back with the same books, yet its comfier to carry due to the padding on the shoulder!
Really happy with the speed of the machine as it really flies in programs and there's little to no delay on opening and closing the office programs, which I did notice on my MBP 13inch with core2duo. The speakers are definitely better on the 15 inch model due to them being on the sides of the keyboard instead of part of the keyboard and the battery life is pretty damn meaty at 7 hours 54 minutes of wireless internet browsing.
Can also play torchlight and day of defeat on max everything at the native res smoothly which is a plus. I'd say if you don't need the speed + nvidia interchangeable graphics card and move around a lot, get the 13 inch MBP. If you need the extra graphics power though or want a faster machine, look at the 15 inch models.