The WiDrive could serve some of Steve's needs as well. Since it comes with fixed storage, there is never a need to format it, so no need for another PC.
The downsides I see are 4 hour battery life the claim. Also, the fixed storage. Even the 128 GB version is limited compared to a the Goflex (but the Goflex is spinning media, not solid state). Compared to the Air Stash+, one needs a cable to use the WiDrive with any PC/Mac...the Air Stash+ has a usb port built in. The Air Stash+ is infinitely expandable.
So, these things all have various trade offs.
Steve -- how do you plan to get your media? Movies, in particular? If you are buying them on iTunes, you won't be able to save them on any kind of external storage, as DRM will restrict your moving files around. If you create your content from your own collection of DVDs/Blurays, then you need a PC. Since you don't have a PC, then you will either buy from iTunes or use Amazon, Netflix, or Hulu (or any of the various TV sites, abc, etc.). So, this means you won't be doing movies on your external storage.
Then comes photos. Clearly, any of these devices will serve that need - more or less. You can use your phone or iPad or camera for photos. If you use a camera, and thus an SD card, then you will need a way to view those photos stored on the card. How will you get them to your iPad? Air Stash can do this because all you do is insert the card from the camera into the air stash (assuming the camera uses an SD card). None of the others can do this. The CCK can, as that is what it is intended for.
Music? You're not going to be ripping CDs since you won't keep a PC. You going to buy that from iTunes? Or Amazon, or Google? Or stream from Spotify or rdio? With Air Stash, you can play your MP3s that are stored on a card in the Air stash, and I suspect you can do that will all of these devices. I am not sure you can store music bought online to one of these devices. The Airstash app can't see the music in the music app on my iPad. So this could be one of thing things with being "iPad only: that could prove limiting.
Other files: If you create these on iPad, then all of these devices will let you storage them as long as the app itself can see the device. I know the Apple apps can see a WebDAV device, but I don't know about the others. Air Stash supports WebDAV with both DirectLink and SideLink (see below). So, I can save Pages, Keynote, and Numbers files to the device as well as read from it.
It seems if you are going to go iPad-only, with a keyboard, and a storage device, that they were a lot of things you have to consider to make everything work and to be able to do everything you need to do.
Personally, I have not done this, so I'm not sure I can say I can see far enough down the road to anticipate all possible outcomes and/or consequences of choices you make now in terms of how you spend your money.
BTW, Air Stash has something called Sidelink, which is still in beta. With it, you can play/stream content from the air stash while also being on line using your wiif. I believe the GoFlex has a similar feature. I don't know about the others. This was not a feature available when these devices first came out. You had to log onto the devices wifi and then you could not surf the internet. I'm using it right now on my iPad, playing music stored on the Air Stash while still surfing the web (not while typing this, though) on my house wifi. It's cool.
Steve, you might want to delay this decision until you can consider this fully. I see various issues that could arise. It worries me to offer advice when I myself have not done what you are attempting to do. I don't know if there are any real 'iPad-only" ninjas around here. Those that are probably don't have much computing needs. For example, if all you want to do is read/send email and surf the web, then things are simple. But when you needs become more demanding, more thought/planning is required.