How strong are your arms? How deep are your pockets? Do you intend to connect it to a larger monitor when you're at home? I think either would be a great machine: I have a Mac Pro at home so I bought a 15" MacBook Pro: it's fine for my purposes. There's no right answer, though. I think the 17" models are a little faster, but I doubt you'd notice much difference in practice.
I also hope to buy a Macbook Pro soon, but not right now. I'm happy with my 13" white macbook, and just got a 20" iMac, so, the macbook became just an on-the-road computer and for tethered shooting. But it's getting old, so, I'm switching to the MacBook pro when AppleCare expires. I'll go for the 15" one because it's enough for those needs. I'd still go for the 15" if it was my main processing machine, but would get a good external monitor over 20" to accompany. In terms of raw processing speed for photoshop, the iMac is slightly faster than the 15" macbook Pro, so, in processing terms, the macbook Pro will be slower than a Mac Pro, but comparable to the current generation iMac, and slower than the future iMacs, which are just to be released ( possibly quad core ). So, I do think iMac/Macbook Pro are a good pair for working.
I actually know a photographer who went to the old MacBook Pro, 17". He loves it, I love it, everyone loves it. But he can't carry it around with any comfort. It's just too big. He mentioned more than once the desire to go for a white macbook just because it's smaller, and easier for location work. On that note, the new 17" macbook pros are not significantly bigger when compared to the 15" ones, in contrast to the older models.