This could probably have been more timely what with the Christmas season just ending, but I've been working on other projects. I've been asked this a lot lately, for obvious reasons, and hopefully I'm addressing it soon enough to get to the post-Christmas crowd.
The first question is what platform you want. The short answer is: buy the one whose platform matches your phone. If you have an iPhone, buy an iPad. If you have an Android phone, buy an Android tablet. The reasoning here is that you're already invested in the app market for that platform, and many (not all) of your apps will be reusable. Some developers want you to use different versions for different form factors, and so the phone versions won't transfer, but by and large you'll be able to make use of previous purchases. You'll also understand the little idiosyncrasies of the system better. Unless you have a specific function that you know only the other tablet has, this is the way to go.
The second question is the form factor. A 7 inch tablet is highly portable, a 10 inch tablet is magazine-like and easier to consume content on. If you don't think you have a preference, I would recommend the larger size. There are more things that it is capable of, and it's closer to what developers are aiming for when they develop for tablets specifically. You should go with the smaller version if you're on a budget and don't need the size. If you're on a budget but do need a large viewing space, consider buying refurbished.
If you are buying an iPad, you're done now. The smaller form factor is the iPad Mini, and the larger is the plain ol' iPad. The newest version of the latter has Apple's "Retina" display, but the older iPad 2 is still perfectly usable if you want to save a little money.
As far as Android, my specific recommendations as of this moment are the Nexus 7 from Google if you want a smaller tablet and the Note 10 from Samsung if you want a larger one. This information changes quickly, but you're probably safe with anything that's called a Nexus, and Samsung is widely acknowledged to make very good hardware.
Things to avoid:
- No-name generic Android tablets abound, but you won't save enough on the cost of the Nexus 7 to justify purchasing something cheaper. They are generally locked to an older version of Android; most of these are on 2.3 or earlier, while Android has reached 4.2 as of this writing.
- I think buying a 3G or 4G tablet is silly for most people, as the situations in which you'd use a tablet but won't be under wifi are extremely rare. With a phone, sometimes you'll want to navigate in the car, or take phone calls or text messages while outside, but unless you're going to do a lot of tablet computing in the park you probably don't need the extra expense at the time of purchase, let alone the data plan.
- The Surface, in my opinion. It's an exciting prospect in a lot of ways, but buying the first generation of a new Microsoft product is almost always a mistake, and Windows 8 has some serious problems.