It depends what is most important to you -- performance or screen size. The new MBPs have significantly improved specs -- RAM can be increased to 3GB instead of 2GB, and the new Core2Duo processors are quite significantly better than the CoreDuos.
So it depends on what you are doing. If you're doing Word, Powerpoint/keynote, internet browsing, then either will work just as well, and you can choose based on screen size and battery life. However, if you're doing video editing, encoding, Photoshop, Illustrator, etc (ie. high end video or graphics), then RAM and processor speeds start to matter.
If you're using Parallels to boot into Windows XP/Vista, then RAM REALLY starts to matter. As someone with 1.5GB of RAM on a MBP who has run Windows XP in Parallels, I can tell you it can be a painful experience switching between them a lot, and 3 GB of RAM would be just wonderful.
Of course, if you plan to use boot camp, then that doesn't matter, and it will not run much faster on the newer machine.
This is my opinion anyway, and it's not a clear answer as it depends what you use it for.
If I were you I would definitely go with the 15" MBP Core2Duo over the 17" MBP CoreDuo in a heartbeat. (btw, he Core2Duos have Wireless N cards, which will save you a bit of money down the line)