Going for an MBA as a 2nd computer. i currently use MBP 15" i7 that i am tired of hauling all day in my backpack. to make it clear, i am talking about the highest end old generation and the entry level 13.3 available. -point of concern. never being able to upgrade RAM on old MBA. new MBA must be ordered with RAM upgrade otherwise forget it. battery life 5 vs. 7? what are real times anyway, i get about 4hrs on my mbp that is supposed to do 7? backlit keys? performance? $300 difference. which one would you get?