The two devices do much the same job, but are designed for use in different ways. An iPad is very handy for situations where you might want something smaller and easier to carry - I use mine on aircraft, and at meetings for example.
You can get wireless keyboards to use with an iPad, and they can be helpful for more serious typing jobs.
It all depends on individual circumstances. Personally, I prefer to have both devices - one for travelling and one for office work. If I have to do any serious report writing when I'm away from my office I take a laptop with me. I wouldn't use an iPad for that.