I was going through the same situation just over a week back. After adding the 160gb (fastest, coolest most efficient drive) to the 15" the price difference between that and the 17" became pretty narrow, making the decision even harder.
The possible downsides with the 17", maybe the screen was too high res, text was bigger and slightly easier to read on the 15" and final cut pro seemed perfectly useable. The wrist area on the 17" felt too big, i'm used to resting my palms on the edge of a laptop and was worried about scuffing from watch and bracelet etc.. but then found Marware Protection Pack, which seems cool and also fixes any possible gap issues, and finally portability - but i realised the only time i ever noticed my laptop size is on an airplane, and i'm not exactly flying around the world every day. So i ended up reasoning, at least with 17" i can resize the resolution and/or make txt bigger within software etc.. and never get that feeling "i wish i had just a bit more screen estate". The 17" is pretty much just an inch wider when closed, it really isn't that much bigger or heavier, if you like at them side to side, still bigger than the average laptop i suppose, but a lot skinnier than any 17" i've seen!!.
but the points i found on the net which helped tip the scale. 8x DVD burner, i knew i'd always be wishing i had this when burning a disc for a friend/client on a 6x, if just to save 10-20 seconds or whatever. I think previously the grphics card on the 17" didn't need underclocking as much as the 15", due to the size and more room for cooling etc.. (hence why there is an 8x), but i think with the C2D neither version is underclocked (??), eitherway the fact that the bigger chassis offered more cooling and the potential for slightly better performance... added to the fact i always secretly lusted over the 17" Powerbook since it was first introduced, but always considered it the ultimate in luxury, it is one of those Dream products, and has a bit more 'cool' factor over the 15", i ended up ordering the 17" which should be arriving here any day now.
If you don't have portability issues, don't mind the shipping delay, and if price isn't an issue, get the 17", if i'm not mistaken they have a stronger resale value than the 15" too!!