I'm deciding between a rMPB 13 and a MBA. I'm almost convinced to buy the rMPB because I prefer the screen. Most of the time I won't need the better batterylife of the air except when I have to study in a library without a poweroutlet. Let's say I sit there from 10:00 to 20:00, with regular breaks. The brightness on 30/40% and running Safari (+- 20 tabs), Word and Spotify. Will the rMPB's battery last?