With the addition of the iPad mini, the answer to this question becomes more complicated. The answer really depends on what the majority of tasks you will do with your new iPad. If you plan on doing mostly consuming of information (reading/watching videos/emails), then I would venture to say get the iPad mini. If you plan on doing more creation-type tasks, I think the extra couple inches on the iPad (4, shall we say) will be worth it. I would recommend against purchasing an iPad 2 at this point, even though it costs $100 less than the iPad4.
16, 32, or 64GB?
In terms of storage capacity, I recommend getting the largest one that you can afford. 16GB will be fine for you if all you do is surf the web, watch YouTube, and have just a few hundred songs to listen to. But if you're like me, and enjoy having a large selection of apps to play with, get the 32 or even the 64. You will not regret it. Apps such as iMovie, Pages, and iPhoto take up A LOT of space on your iPad. It does get quite stressful when you are running out of space, but still need the space for additional photos, videos, music and apps. I experienced this when I had a 16GB iPhone 4S. Since I very much enjoyed taking photos and videos on the phone, I found myself always having to transfer them over to the cloud, or to a backup hard drive.
My wife has enjoyed her 16 gb first gen iPad since 2010, and has never had to worry about running out of space. She does not listen to mp3s on it; she does not watch videos on it. She simply uses it as a browser on Safari and reads the occasional book on iBooks. She reads a multitude of blogs every day. So for her, 16 gb is plenty. She does not need the larger capacities because of the style of her iPad usage. I also knew someone who was proud to own a 64 gb first gen iPad. One year later I asked him to check how much space he had used on it: only 4gb total! That upgraded purchase for him, in my opinion, was a waste of $200.
iPad mini or iPad 4?
I have read a lot of reviews on the iPad mini. The large majority have been overwhelmingly positive. When Apple first announced it in October, I was skeptical about how it would do. I thought that there was a chance, since it was announced as a non-Retina display, that it would be a huge disappointment. I was wrong: I can't wait to purchase one for my family. It would come on all family trips with us. We would be able to leave our larger, much heavier iPads at home. Plus with the fact that the iPad mini boasts the same front and rear camera specs as the iPad 4th generation, I am looking forward to getting possibly the second version of it. I'm hoping (and expecting) it to have a Retina display, but who knows?
Bottom line is, if you have never owned a device with a Retina display, you won't notice any difference. If you have, you might find yourself staring at the pixels.
Get the iPad 4 if you plan on doing heavier tasks (gaming, content creation) and get the iPad mini if you plan on carrying it with you daily and if you enjoy lighter tasks (surfing the web, FaceTime, etc.).
Both are great devices. You won't regret making the purchase. As a teacher, as mentioned previously, if you plan on just referring to the iPad as a PDF reader for manuals, the iPad mini will be just fine. If, however, you plan on showing and presenting the screen to your students, or creating materials using it as an interactive whiteboard, the larger iPad is for you.
Check out this article by Daniel Edwards, which might help you in your decision.
Wifi only or Wifi + Cellular?
Here in Canada, we have the luxury of being able to tether our phone data (create a wireless hotspot) over to other devices for free. Therefore if you have a smartphone with a data plan, you would never need to get an iPad with 3G or 4G. It would almost be pointless, paying for data twice over when you didn't have to. It does take a couple taps on each device each time you want to connect, but I think it's worth the small nuisance. I met someone a while back who was proud to say that he was in fact paying for data for his iPhone and his iPad. He was working on his iPad and told me that he just thought it was too much trouble to set up the tethering each time. He has too much money to burn.
Tethering in the United States is, to my knowledge, a little more complicated. Each network will have different options for tethering, but might require you to sign up for their biggest data plan. Jailbreaking your device can allow you to freely tether. (*Americans, please correct me if I'm wrong in this regard.)
So, if you do have a smartphone, I would say it is not necessary to get a 3/4G iPad. However, if you just have a regular flip phone sans data plan and you perhaps have a long daily commute on a bus or train, having that data plan on your iPad might not be a bad idea. Also, if you are a constant traveler, you can purchase sim cards for your iPad in whichever country you are visiting.
Please let us know in the comments what other factors you would consider in choosing the right iPad for you!