One thing I'll say is that you don't need VPC to run MSN messenger. But you may need it for yahoo. I have MSN on my iMac and it works fine. It's the Mac version and it has most of the features of the windoze version. But I'm not sure on Yahoo, i've never tried d/l ing it, but I do know that Mac's work with Yahoo games chatrooms and I also know that the IM system is the same thing. So i'm guessing there is a mac-compatible Yahoo.
And portability would definitely go for the 15 incher. Most companies make their Laptop carry cases for the 15 inchers and you would have to special order one for the 17 incher. but G5 is right, there really isn't much difference between how you would use them.