Should you want to buy an iPad, buy the 12.9″ model together with a Pencil. The Pencil is the precision tool replacing the mouse on an iPad. It is superior for almost any task, most obviously for drawing, but the precision comes in handy for object manipulation surprisingly often.
It has allowed me to move all my diagramming to the iPad, thanks to the excellent Omnigraffle. Paper by 53 has replaced pen and paper for exploring ideas. OnShape shows that you can implement a professional CAD system on the iPad. PDF Expert delivers excellent tools to annotate documents, and Pixelmator makes it easy to combine graphics to illustrate a point.
People are underestimating the impact of the Pencil, because it requires you to use your iPad flat. You can either use your iPad with the Pencil, or with an external keyboard, but both together is awkward. People seem reluctant to use the iPad naked. But give it a try. The Pencil is a huge improvement whenever you need to work with images, an external keyboard is only a small step up from typing directly on glass. And with speech recognition getting better, we will soon reach the point where even nonnative speakers will be able to dictate faster than they could have ever hoped to type. An external keyboard offers familiarity, but it is also blinding you to see how to use an iPad most effectively.