The 16 GB model is plenty unless your music library is very large or you want to download (and keep) a lot of movies or have hundreds of apps. Pictures don't occupy that much space and you can simply copy them to your iPhoto Library and delete them from the iPad if you need more.
There is no firm rule, it all depends on what you want to do with it.
i only have a thousand songs and more or less 80 apps. and a 1.2GB worth of books. I just wanted to have at least 2GB free for pictures (not much of movies,,,) thanks
It's all relative, 1000 songs is perhaps 5 or 6 GB or so right there, and to me 80 is a lot of apps. I'd guess you're using on the order of 10 - 12 GB already. In that case 16 GB goes from "plenty" to "adequate"
I'd recommend the 32 GB model. You're only going to want more songs and apps.
Capacity is a personal choice and only you know how many songs, movies, photos, ebooks and apps you intend to use on the device.
If you intend to use the iPad primarily for web use and e-mail and only want to have a modest number of songs, photos, video clips and apps, or price is most important to you, 16 GB should be adequate. In general, however, you likely will be most pleased buying as much capacity as you can afford as it cannot be upgraded after purchase.
Even 64 GB may not truly satisfy those who want to store -- or particularly those who want to create and edit -- hundreds or thousands of songs, photos and videos, particularly given that the high-resolution display dictates the need for much larger image, video and app files than earlier iPad models.
How much content will fit on my iPod or iPhone? (Also applies to iPad)
How much space is your Other using? You may be able to reduce.
How Do I Get Rid Of The “Other” Data Stored On My iPad Or iPhone?
With an iOS device, the “Other” space in iTunes is used to store things like documents, settings, caches, and a few other important items. If you sync lots of documents to apps like GoodReader, DropCopy, or anything else that reads external files, your storage use can skyrocket. With iOS 5, you can see exactly which applications are taking up the most space. Just head to Settings > General > Usage, and tap the button labeled Show All Apps. The storage section will show you the app and how much storage space it is taking up. Tap on the app name to get a description of the additional storage space being used by the app’s documents and data. You can remove the storage-hogging application and all of its data directly from this screen, or manually remove the data by opening the app. Some applications, especially those designed by Apple, will allow you to remove stored data by swiping from left to right on the item to reveal a Delete button.
I got the 16 GB because 1) it was the only one available in the store on launch weekend and 2) I wanted to see what the iTunes Match and iCloud are all about.
I have quite a few holiday snaps from over the years that I would like to have on my iPad, but overall they took up about 6 GB, so I've had to be a bit more judicous in my choices.
So far the cloud and match seem to have helped with needing to keep the space requirements low for the content that they can work with.
In my case if I had it to do all over again I think I would have wanted the 32 GB. I have the 64GB phone and it is only about half full.
My old iPad was 16gb and I managed pretty well with that, but chose to move up to 32gb with the new one. No regrets. Although I am not currently using the full 32gb (at about 60% capacity as I write this) I would have maxed out a 16gb by now, at least in terms of having to pay more attention to content and file management. I've actually used the camera a bit more than I would have thought, store more video and general multimedia content than previously, and have the capacity to carry more with me when I know I may. Not have wifi access. I have somewhere north of 16,000 music files in iTunes, almost all of that in iTunes Match, much of it uploaded (a lot of obscure stuff) at 320 Kbps, so it wouldn't take all that much to push the content up to capacity.
Get as much as you feel you can afford or justify against your budget. Better to have extra room than run out, IMHO.