Denis,
My rule with Macs -- always buy the newest, because it extends their life out. And if you don't something will turn out to be out of date and it'll bite you in the butt. Buy the new 17" with black keys, and with NON GLARE monitor ($50 extra), and be done with it. Once you get in the middle of the job, you'll appreciate the tiny amount of extra space that the 17 gives, over the 15. If shooting vertical frame, the preview is quite small on the 15. Fill it with RAM, and buy the 7200rpm drive. Write the check and be done with it, knowing youve made a good long term investment. It'll also run a 30" monitor, if you buy the duallink adaptor too. Nice when living in tight spaces.
The only downside for me: the viewing angle is severely restricted with the 17 non glare: you get even a tiny bit off center and the frame looks like it's 2/3 of a stop brighter. Can fool you very easily.