Go for the 17". You won't notice a difference between the two speed wise. Unless you want a full 4 Gbs of RAM and 70 Mhz more processing power (insignificant), just get the 17".
It really depend what you need the notebook for? Do you need the notebook for mobility or do you need it to be a home notebook. I do believe that the 2.4ghz 15" will be better in the long run, but not significantly. The difference in ghz between the two processors is 0.7 ghz? So the choice is up to you really. Ask yourself the question about what you need the notebook for and then tell me in the thread. Personally, I'm going for my first mac, when leopard arrives, and I'm going for the 2.16ghz macbook not the 2.0 ghz macbook as I like a bigger processor for speed.