Alternatively the 13-inch may be the model for you. You may be swayed by the SD memory stick slot for example. At the end of the day it only costs £100 more to upgrade to the bigger screen and that extra screen space may make all the difference. That said, you could always spend some of that £100 on an external monitor and plug your 11-inch MacBook Air into that.
For some the decision may be between the 13-inch MacBook Air and the MacBook Pro with Retina display. The 13-inch Air has a native 1,440-x-900 screen resolution compared to the 2,560-by-1,600 resolution on the 13in MacBook Pro with Retina. A superior Retina screen will cost you another £120 on top of the price of the high end MacBook Air if you opted for the entry-level MacBook Pro with Retina.
Where in the case of the 13-inch MacBook Air there may be a decision to be made between choosing the Retina or Pro 13-inch versions, in the case of the 11-inch model there is no real equivalent in terms of laptop. Perhaps the only competition to the 11in MacBook Air, from Apple's offering at least, is the iPad. You can buy a 128GB iPad for £739 (saving £110) and that has cellular connectivity so you can always be online.
It's a matter of personal taste as to whether the 13in or 11in screen is best, we'd advise a visit to an Apple Store to try both out. Unfortunately you won't be able to put one in your backpack in order to judge how heavy it is.
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