An 11.6" MacBook Air comes with a 1.6GHZ dual core Core i5-2467M and a 128GB SSD for around £1000. The 13" MacBook Pro comes with a 2.3GHZ dual core core i5-2410M and a 320GB 7200RPM hard drive for around £1000.
I suppose screen quality might be another factor:
http://www.anandtech.com/show/4528/t...3inch-review/7
The 13" MacBook Pro has a slightly better screen.
However,if you are serious about image editing you will need to spend a bit more on the matte screen option but you can only get this with the 15" MacBook Pro.