Hey,
You really need both. An iPad, as much as I hate to say it, is not yet a replacement for the laptop. Give it time, when productivity on the product gets enhanced. Then maybe, you could use it a little more like a laptop. But right now, its not cut out as a laptop replacement and shouldn't be considered as an alternative.
Also, don't get the 1st generation. The 1st gen will always suck in hindsight when the 2nd gen comes out.