Fundamentally there are only two versions of the iPad: one with just Wi-Fi, one with Wi-Fi and 3G. After that it's a simple choice of deciding how much storage you need/can afford: 16GB, 32GB and 64GB.
We can think of no better way of representing this than a graph, so that's what we've created. Here you can see that the cheapest 3G model is more expensive than the 32GB Wi-Fi only version, and that the gap between the 32GB and 64GB versions is slightly larger than that of the 16GB and 32GB. Aside from demonstrating how expensive the 3G version is, though, this only tells part of the story. Ultimately, which iPad you should buy depends on how you intend to use it.
All I really want is a big iPod Touch
If this is the case then it should go without saying that a Wi-Fi only iPad is your best bet. Considering the iPad lends itself well to video playback, particularly of the HD variety, the 16GB version seems a tad on the small side. That leaves the 32GB and 64GB edition, and it's the former we'd go for. Frankly, £600 for an iPad is just a scary amount of money, though if money is no object then it seems trifling to refuse.
I'm into reading books, magazines and the internet
If you're more into reading and sharing information than watching video and movies, the 16GB iPad makes more sense. Here the potential storage limitation is unlikely to be a problem, but whether to go Wi-Fi only or Wi-Fi + 3G is a trickier question. If accessing the internet from anywhere is important to you then the 3G version is a no-brainer, but if you've got a smartphone and a contract to go with it, is it really sensible to spend even more on 3G data?
I want to ditch my laptop
If this is your rational for buying an iPad, think very carefully before you do. As we found in our iPad review and discussed in the podcast, there are numerous reasons why the iPad won't replace a laptop. Not only is it awkward to work on when on the move, its productivity applications are far from flawless - not to mention the fact you need a PC or Mac to sync data! An iPad can complement a laptop, but as yet it can't replace one.
I thought about buying a netbook, but they're too cheap and nasty!
While the iPad won't replace a proper laptop, as an (albeit expensive) alternative to a netbook it's far more palatable. Thanks to the customised OS it's actually faster to use than a netbook, and unlike many netbooks it can play 720p HD video pretty comfortably. It's more than a match for writing emails and the like and even without flash support, the screen and speed make it a superior web browsing device. It still can't perform some of the tasks a netbook can, but for consuming content on the move it's in a different class.
As to which version to buy, if you're planning to have it with you everywhere then a 3G version may be worth considering. Ultimately it comes down to how deep your pockets are.
My life won't be complete without an iPad
If you're convinced that the iPad is the answer to your prayers: don't hold back. You'll need either a 32GB or 64GB iPad 3G so you can pick up email on the move, browse the internet and generally make the most of the iPad's potential. You'll only regret it later if you go for a Wi-Fi only version.