About everybody say that mobile devices are the future of personal computers, since they've started to replace (at least in some way) the PCs. For now the tables are... everywhere. There are a greater number of options (in the stores and over the internet), so it gets harder to choice the best tablet you need.
So you must have some criteria:
Do you need one? What for?
It's the questions you should ask before deciding to buy one. If you only want to read emails or browse the internet you don't need a device with high processing power. Some people just want to read some quality books, but the touchscreens of their smartphones are too small and the notebooks are to uncomfortable - in this situation just use Kindle (it works best with Amazon, so you also have the chance so search for online books).
Other people want to watch some movies or play games while they travel by train or by plane and, again, they find uncomfortable to use netbooks or notebooks. Watching movies needs some processing power, so you search for tables with some available resources and, most important, a good battery - something that keeps you stick to it at least three or four hours working on full power. This year, not many weeks ago, was introduced a portable generator for mobile devices that, eventually, will replace the current batteries, but it will take some time for that.
The price range
The cheaper tablet costs a little less that 200 euros, but the price depend on where you are on the globe and where you buy from. The top two Apple's contenders are iPads and iPad mini $358.20 at Apple Store, while Microsoft Surface costs $699.00 at Microsoft Store tablet.
The operating system
Apple's iOS is theoperating system on the iPad and iPad mini, Google's Android has many hardware choices from the likes of Acer, Amazon, Asus, Barnes & Noble, Google, Samsung, while Windows RT runs on Microsoft Surface.
When trying to decide the best operating system for you find out that a version of your favourites applications have been developed for it. There are small chances no chances to get an OS with no apps (such as movie viewers or games), but people are slaves to their habits: once you have the tablet but you'll want your favourites apps to run on it.
The brand
People have the tendency to stick with the same brand of products if they know the quality is high, especially from their own experience, or if they love some product or another. For example, an important part of Apple's sales come from people that have been using their varios products for years. Other percent of sales come from word spreading around... about how cool the iPhones and iPads are...
Some other important things you should pay attention are the accessories, time since last refresh, and other features you desire. If you own all Apple products and have a large iTunes library it will make little sense to switch to an Android based tablet. If you have a lot of ebooks you should stick to an Kindle or something of that nature.