There are many things that factor into play when choosing your tablet.
The most relevant technical specification is pressure sensitivity. The more, the better.
Smaller tablets are better, cheaper, easier to fit into your desktop, and they work just like the bigger ones. I would only choose the big model if you're going to use it for CAD.
Think about the use you'll give to your tablet. If you want if for painting, coloring and photo manipulation, it will be 100% useful. For drawing, only Cintiq models are really good, and for designing, you need no more than a mouse.
There are second hand markets or brand new old models. They are really worth checking.
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