There is a 128GB version available, but it costs at least an eye-watering £639. That seems like an unwise purchase to me -- at the rate tablets are changing, a 2013 model will probably be a bit sluggish in two years' time. I'd recommend buying the cheapest model you can get away with so if you end up replacing it in a couple of years you won't feel as fleeced. 16GB will probably do if you have a computer handy to transfer any large files onto once you've dealt with them, although that will mean lots of faffing around with iTunes.
Do you need 4G?
Both the iPad Air and the new iPad mini have 4G models available. That's lovely for those who need it, but have a serious think about whether you do. It adds £100 to the price: my experience when I bought a 3G model a few years ago is that I didn't use it very often, relying on coffee shop Wi-Fi and my smart phone for connectivity instead, but that won't be the same for everyone.
With lots of other tablets coming along with amazing screens and music/video stores, one thing Apple is keen to push is that this is not just a tablet for fun, it's also for getting stuff done. You now get free copies of Apple's version of Microsoft Office -- Pages, Numbers and Keynote, plus movie-editing app iMovie, photo editor iPhoto and music-maker Garageband.
I don't feel like these apps are yet at the stage when you can throw your laptop away and just use an iPad, but it's clear that's the direction we're heading in. See them as a nice bonus rather than a reason to rush out and buy an iPad Air right now.
Apple owns the tablet category for good reason, and the iPad Air helps it stay out in front. Yes, it's more expensive than the competition, but if you can, it's worth shelling out for a more polished product, and that's the Air.