MacBook Air :: Variation On A MBP Theme?

I have a 27" iMac with a near top configuration. I use it for a large part of the time in a business context, and for that I use Illustrator and Photoshop CS5 extensively. After a difficult conversion from WindowsLand - my fault, I'm sure - I'm now a very enthusiastic Apple customer. The rest of the time is email, on the Web, hosting a large family photo library, music and all the usual stuff. 

I also have an iPad 1st generation, which I use for email and as a storage for my digital photographs when I go on long haul vacations, two or three times a year [family live abroad]. While the iPad is great, it doesn't have ethernet, and not all places have WiFi. Also, photo management and linking to social media sites is a bit of a kludge. It doesn't run MS Office for Mac either. 

So I am considering buying a laptop for use when I'm not in front of the iMac. I have ruled out the 15" MacBook Pro on size and portability grounds [and the new retina model on cost], so I'm looking at the 13" MacBook Pro, primarily because I had assumed the Air would not be sufficently powerful or capable for my needs. Now I'm in a quandary. Which might be the best option - MBP or MBA? 

I like the MBP because it is upgradeable, whereas the Air isn't. However, I'm not replaceing the iMac with the laptop, simply wishing to be able to do a small amount of support work for my colleagues when I'm away. If I buy the best configuration for my needs to start with, why would I need to upgrade it? In addition, both the MBA and MBA can provide the ethernet that the iPad can't [albeit with an add-on for the Air] and can deal with the output from my digital cameras better than the iPad can.