That depends solely on what needs you have.
Is mobility important and do you already own a desktop?
I currently own a Lenovo X200 which I'm planning to sell after I get either of the setup !
I'd prefer the mobility of cos when I have to take it out....the 17" MBP would be too bulky and heavy to lug around !
Pick the iMac+MBA as mobility is your first choice. Lenovo X200 is a laptop with only 12" screen with 2GB ram as default (correct me if I'm wrong). MBA has a comparatively larger screen and similar weight. I think not much people will bring along their 17" MBP on the road.
However, MBA is due to update. Do you think you can wait?