i think it depends on what your portability needs are. The mbp really does need quite a large bag (especially when in a second skin or something, and perosnally, you have to be super careful withn your bag too),and so is portable for the daily commute to work, or moving around the house etc, however if you want something with you for most of the time, its footprint is really too large and its also too heavy.
The air has a smaller footprint, but would still restrict you to having a backpack or proper bag,however its weight makes a massive difference.
For me,there hasn't bee a true portable since the 12" powerbook.