If you want portability, get an 11". Maybe you think you need those extra 2", but I believe in time that you will find you are wrong, and never look back.
Two weeks a go, I purchased a MacBook Air. I leaned toward a 11" but found the 1366x768 resolution slightly too dense for the screen (135 ppi). I ended up getting the 13" MBA, which has a 1440x900 screen (128 ppi). This machine I can use at its native resolution, whereas with the 11" model I'd use it at a scaled down resolution.
Before, I had a MacBook Pro with a HDD. I would use my iPad whenever I could, because it'd be instantly on and feel faster in use. My MBA has a 256GB SSD and it feels just as fast as my iPad.
Back to the main topic: I keep Windows VMs, version management and backups at a dedicated server at Leaseweb. Local copies are kept on the MBA and a Mac mini at home (where it's also backed up to external disks by SuperDuper). This way, I can always choose to work on my iPad or MBA, files are accesible from everywhere.
