Apple has 70% of the market and is the undoubted all round leader ( in my views of course). I've had an iPad1 since just after they came out, device works brilliantly and changes the way you will use "a computer". My usage is very similar to your intended use, web browsing, email, content viewing especially on flights, document reading/creation (small doc, long emails), some apps but they are secondary
The downside is you have to pay a relative premium price for it, as they sell out almost immediately new stock comes in you cannot say apple have got the pricing wrong. 100 million sold since launched.
Everyone else is playing catch up
As budget is a factor and given your requirements then as above a Nexus running Anrdoid is a very compelling alternative. A small memory sized apple might be insufficient if you want to keep a few movies on it for a family holiday for example