I have access to a 16GB iPad, 11" MBA, 15" MBP and a 21.5" iMac. They've all got their uses (well, except the 15" MBP which I really need to sell, but I'm not sure where- worried about getting ripped off!), and I wouldn't be as productive without 'em!
I'm an HCI student, and I'm currently traveling 2 hours to and from London each day. When I'm at uni I often require use of a computer- workspaces and computers are really not well provided for at UCL, so I usually end up taking my MBA. The MBA is also absolutely necessary for working overnight when I have a hotel or stay over with somebody.
An iPad wouldn't really be suitable for this- they're far too limited for media creation, multitasking, and lack a tactile input. I see many of my colleagues trying to do work (creating presentations, user-testing, etc.) on ipads, and it often looks like they're spending more time playing with stuff than actually getting work done & being productive... They'll defend them to death though, so they're presumably happy with the situation!
Even with the small screen on the MBA it's very possible to use it like a "normal" computer or laptop, especially when you make good use of full screen apps and spaces. However, it's a complete pain to use the MBA on public transport- I often don't feel comfortable getting it out on the train, and its impossible to use a laptop whilst standing. So, I often borrow the iPad (I have a Touchpad too, but lack of a good pdf reader means its not so useful for me), mostly for use on public transport, when I've not wanted to carry the laptop, or whilst in bed. My course requires LOTS of reading, almost all of the material is distributed electronically, along with many of the textbooks.
The iPad, combined with an app like Goodreader is great for making the best use of time on the train/ in bed/ anywhere I feel comfortable to get it out. Apps like IAWriter and sketchbook also mean that if an idea suddenly pops into my head I can quickly type or draw it up. I also have other stuff on the ipad for when I'm not feeling productive- the Guardian app is pretty good, Instapaper is useful, as is twitter, etc.
The iMac lives at home and is a bit of a luxury, it makes working and procrastinating considerably easier- the sandybridge quads are ridiculously quick, and a bit more screen estate really helps! When I have time I do a pit of photography & the screen isn't idea for that, but otherwise I love it. If I didn't have the iMac at home I'd have got the 13" MBA or kept with a 15" MBP instead of the 11".
However, I'm not exactly a typical user, so much of this may not be useful... My mum is quite happy using an iPad for most of what she needs, so you really need to think about your particular use of the devices.
I tether using my phone so have the wifi only model, and 16GB has never been restrictive, but I only really have books, articles and papers on the iPad. If I upgrade to the next model with the double-resolution screen I'd likely get 32GB, as it'd be useful for using with Photosmith in the future.
Now, I'd best stop procrastinating and get some work done, or I won't have any time to go riding before term starts...