Always get the MacBook Pro. It's vastly superior.
Pro: FOUR GIGS OF RAM (hello, grab it!); fast processor; more than adequate hard drive that can even be replaced with a Western Digital (the best brand, also used in Macs) 320-500 gig for $90 on newegg.com; very comfortable screen size; PHENOMENAL speakers (seriously, the clarity is something you'd expect from Bose surround sound speakers); that price.
Con: None to speak of. Maybe the price if you were buying it new... but even that's not an issue here.
Pro: New, cool trackpad, fast processor, very portable, attractive, easy access to replace the hard drive and upgrade memory.
Con: Tiny screen, woefully crappy speakers, aluminum finish wears away very easily, updates are coming very soon so you'll get the older version, small hard drive (though easily and inexpensively updated), average amount of RAM, and did I mention awful sound quality?
My vote is always for the MacBook Pro. I've had mine for 2 and a half years and it's still better than most of the PCs out there. I've upgraded the hard drive and still have way more than enough memory with only 2 gigs installed. I'm not a huge music fanatic but I'm so impressed with the quality of these laptop speakers. It makes me respect the computer even more. The MacBook Pro is also a more professional grade laptop than the MacBook so it can handle a huge amount of activity that the new MacBook still can't. That's not to say the MacBook isn't more than adequately adept -- it's just that the MacBook Pro can tackle literally 250 web browser tabs, Word, Adobe Acrobat, Mail, Preview, and Photoshop all at once (and that's on my laptop with only 2 gigs of memory). If that sounds extreme, I assure you I'm not exaggerating. I've done this in the past and will likely wind up doing it in the future. My good friend's brand new MacBook just can't handle that extreme load. However, most people will never need a machine to do that much at once.
Get the Pro.
Mac #5 is a 17" MacBook Pro.