I find 11" laptops to be just a tad bit small for everyday usage especially if it will be your only computer.
The one advantage the 13" MBA has is it has a better battery which gives it a very long usable time between charging.
In terms of weight there isn't really a significant difference between the 2.
In terms of pricing it isn't that much more between the 2 and if you buy it through the Apple Higher education discount you do get a 20% discount or if you so happen to be going to Luxembourg you could buy it for 23% less than you would in UK.
Finally changed over to using a Mac laptop, bought a MBP 13" and did weigh it up against a MBA 13" with the 256GB storage.
One thing you might want to consider is buying a unit that has more storage and memory to future proof it.