Neither.
I use the iPad to browse the web, read PDFs, use epicurious cookbook, check Google maps, look at stocks, check local weather, watch podcasts, show youtube clips to people, play some games, watch movies/netflix.
I will use the MBA for programming, blogging, word processing, note taking, VNC, remote management, access portal to servers, video skype/IM, filing taxes, processing orders/reciepts, spreadsheets, light video/image editing, light gaming and other OSX apps that I want to run that I couldn't on the iPad
I use the Mac Pro for 3D modeling, animation, rendering, HD video editing, video encoding, large scale Photoshop projects, archiving, bluray ripping/burning and high-end gaming, data server, OpenCL and mathematical analysis.
To break it down regarding my computer usage, I'm 70% on my iPad 30% on my Mac Pro. Adding the MBA cuts my iPad usage in half, delegating optimal productivity to each of these devices. In turn, I save energy by not wasting CPU cycles on my MacPro just to surf the web.