I've pretty much had only an iPad since the day iPad 1 came out. Have a desktop machine at work, but that's just for work (programming), occasionally used for those few things I need to do which the iPad can't. There is a PC at home, but it's so decrepit it serves little purpose beyond iTunes host and storage for old files. I take the iPad everywhere. Got an iPhone just a month ago. iPad is very capable, but its limits are real: while we're getting away from managing files per se, there are still an awful lot of them around (in need of transition to the "no apparent file system" paradigm); the refusal to allow on-tablet software development is a critical limitation (yeah it's not meant for it, but the capability is entirely there stifled only by policy).
Finally gonna break down and get a "real" computer. Been eager to get a 27" loaded iMac for a year, but the money just isn't coming together fast enough and I'm pretty much at the "must buy now" point. Looking at a refurbished low-end 13" MBA now: cheap (as far as Macs go), bigger screen won't compete with iPad's 10", SD card slot makes upgrading storage easy (64GB microSD in a flush-mount adapter), runs Xcode, light, tough.
The key in asking "iPad and/or MBA?" is recognizing they are, in fact, different. iPad is "anywhere, anytime", as small as sanely possible (iPhone too small) yet still a good-sized screen and has always-on cellular data; it's limited, but what it does is about 80% of what many/most use a computer for. MBA takes that, expands the screen a bit more, has faster input, supports file systems, and allows coding ... but loses the extreme portability and "always connected" ability.
With a little adjustment, the iPad can handle about 80% of all tasks. I've pushed that almost as far as I can, but it just doesn't cover the remaining 20% - much of which is more a matter of legacy (file systems) than capability.