I've used both machines, and I can say the the 15 incher you may seriously want to look at. The difference between the two machines are greater than just the screen.
For most users, the MacBook should suffice. However, in terms of working directly on the laptop, obviously the 15 inch screen makes a huge difference. You can actually do some serious work on the screen. To put it in retrospect, the MacBook Pro has the same screen real estate as a 17" iMac.
I personally prefer the feel of the MacBook Pro's keyboard over the MacBook, but every typist is different. Also worth noting, the MacBook's keyboard will not touch the screen upon closing, unlike the MacBook Pro. Thus, if you type with greasy fingers, you'll see it on the screen of the MacBook Pro (no Big Mac and ehMac surfing at the same time

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Robustness. Hmm. Typical of all aluminum notebooks, the MacBook Pro does suffer from slight warping once in a while. Obviously the plastic MacBook will not (however, I've seen a MacBook's trackpad bubble. Go figure.). However, I still find the aluminum wayyy sexy compared to the MacBook. Yet, I must admit, the black MacBook is very attractive.
Memory requirements, in my opinion, should be higher with the Intels. This is because, if you are using ANY PPC only apps (ie. Microsoft Office), they will use significantly more memory running on the Intel than on the PPC based Mac (because of Rosetta translation). Thus, 1 GB would be the bare minimum.
The one clear advantage of the MacBook Pro would be its graphics card. AFAIK, Aperture does not run on the MacBook (at least, not without some hacking). Obviously, any graphics related tasks will be significantly faster on the MacBook Pro over the MacBook.
A few notable things that the MacBook Pro has over the MacBook:
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ExpressCard slot. While there might not be a lot of accessories using this standard at the moment, many are on the way. This replacement for the PC Card will allow enhancements such as FireWire 800, EDGE (for portable internet), External SATA, and more. It makes this machine quite expandable.
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Full size DVI port. May not seem like much, but it really sucks when you have to do a presenation and you forgot your video adapter. Or perhaps (as is the case with the MacBook), you might not have bought one yet.
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Light sensitive Display. The the light sensor is nice to have as the daylight changes, or as you move the laptop around.
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Backlit Keyboard. Even nicer than the display would have to be the fact the keyboard is backlit. The soft white glow is especially handy when working in dark environments. Another thing of note: The keys on the MacBook Pro are laser etched, after the paint is applied in the manufacturing process. The MacBook has it's lettering applied as another part. Thus the keys on the MacBook Pro are much less likely to wear off.
One thing that comes to mind immediately when I think of advantages of the MacBook versus the MacBook Pro, is that the MacBook has a
removable hard drive. To be more clear, the hard drive is user-accessible, which in the MacBook Pro, it is not.
Also, the MacBook is obviously smaller, and because of its case, perhaps more durable.
I suggest going to your local Mac dealer and playing with the two machines. Open up some apps, and see how you like it. Don't be afraid to spend time deciding on a large purchase like this.