The 15 incher should suit you fine. The size difference between the two is negligible enough to not really bother any normal human with its size/carry weight. The max weight is like 6 pounds for an Apple Powerbook. Thats a pretty good weight for any traveler. So pretty much the only factor is price. Both have more than sufficient performance to run anything youve described.
About Virutal PC - yes you do need a full copy of the Windows OS to run it. But tihs does not bog down your performance like you think it does. Instead Virtual PC just acts as a sort of middle man between the Mac components and the Windows OS that is being virtually run. Performance is crap on no matter what machine you run it on, but it should be adequite enough to run MSN through. Virtual PC is packaged toinclude a copy of the Windows OS (I believe it is XP home, though if you already own Pro that can be used without any problem). If you need help installing anything on Virtual PC if you ever get it, Ill gladly help you out (Private message me).
The 15 inch screen fabrication process hit snags early on with white spots and dead pixels. This has been almost completely sorted out and I havent of these problems occuring for a solid couple months.