Unless you do a lot of graphics work on the road, skip the 15 (my opinion). For the money, I'd much rather have the 13 + large external display + keyboard + mouse. Make a much larger, more functional setup for when you're at home, while having a perfectly reasonable display for the road. Waaay better use of your money IMHO.
Oh, and I agree with the others. Wait on iPad for Rev 2. 9.7" of FaceTime would be awesome!