as someone who owns the air (13") as well as an ipad and 2 ipad minis I would say the air is better for use in bed. Screen is vertical and doesnt require work to keep it that way, backlit keys, battery life is better than the iPad and ipad mini, its perfect for this use. They all will hold their value ok but the more you spend the harder the sell is, the more you spend the fewer people will want it. I buy and sell macbooks all the time and make $50-150 per easily. Just sold a retina 15" that was 3 months old, mint condition, 16gb ram 512gb ssd, 2.7ghz quad. The computer goes for $2799 plus tax from apple, I bought it for $1650 and sold it for $1850. Whereas if you look at mac minis people are getting $500 for a two year old mini that sells for like $599 brand new. Two year old base 21.5" imacs go for 900-1000 when they are like $1300 new. The cheaper models will always have higher resale potential. The resale on the retina is horrible because its too expensive. Same will be true with the maxed out air. That said I think the 13" is the better computer by far and will also hold its value better. 16:9 screens suck, and the battery life of the 13 is so much better, plus full size trackpad and sd slot
