I have been using the 17-inch for almost two years, before that I had two 15-inch in a row. In this case I purchased the 17-inch solely to get the Express-34 card slot, so I could attach eSATA devices with the appropriate card. Those things are Fast!
Not much difference in usable screen real estate between the two computers with the hi-res matte screens. Either is usable for Photoshop or Lightroom - but then I've used both of those apps on a 13-inch with some success, so maybe I'm not the best source on this.
The 17-inch is a beast, no question, though it's only an inch larger in each dimension than the 15. I use it on my lap all the time.
Now, all that said -- if I had a photo editing workstation at my office, and wanted a laptop for travel and basic editing, I would get a 13-inch Macbook Air. The weight savings are significant, the screen is very good (just be careful of reflections), and it's plenty fast enough for basic work. Heck, even the 11-inch would work, I just ran into a former colleague at a football bowl game, he was editing everything on an 11-inch MBA, said it was great to always have it in his camera bag.