I had the same consideration.
If it is just for reading webpages, giving one sentence remarks/replies, youtube, video, go for the iPad 2, hang on the couch, be happy with it. You'll have the iMac for serious work.
If you want to be able to type larger emails, make reports, minutes, excel calculations, you can consider a bluetooth keyboard to go with the iPad, but the weight/wieldyness (is that even a word? well, what ever, I'm foreign

) swings to the MBA 11".
I have an iPad 1, mba 11 i7, MBA 13 and a year old MBP 13 with SSD.
I plan to sell the iPad and the MBA13" and keep the MBA 11" and MBP.
The MBP will be in the office permanent as a backup and serving for heavier work like video encoding.
The 11" will be used for every day chores, including hanging on the couch and reading some papers.
I've tested all of them extensively and found the weight, size and performance of the MBA11" really good. The iPad (for me) lacks a file system and keyboard. I know that is not it's function, but I need that.
I now have a package that is lighter than an iPad with wireless keyboard, easier to pack and carry around and I have every function a laptop offers.
As I don't have a large screen, I plan to connect the 11" to a Thunderbolt display, that way giving me plenty of real estate when I'm at the office. Selling the MBA and iPad will fund that 27", I hope.