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I would save up another $100-$200 and get a 15" PC notebook. In the meantime, I write with a pen and paper (I do this now even with multiple computers).
 
I've just finished adding 1200 pages to a multi volume novel using primarily my iPad with a Bluetooth keyboard.

Never before has portable authoring been this easy and this much fun.

I use Storyist and do also tie back to my Mac on occasion, but for portable writing, the iPad is a dream machine for me.
 
The iPad is a surprisingly capable device for writing if you're on the move a lot. I've written tens of thousands of words on mine. The combination of astonishing battery-life and instant-on means you can use any spare time while out & about to write a few paragraphs.

That said, I would never have it be my only machine. I heartily endorse the recommendation for Scrivener, which you can run on either OSX or Windows. I'd get the machine with the biggest screen for your budget.
 
iPad in a heartbeat. You'd be getting a current, best-of-breed device that is more than capable of handling what you throw at it.

Your alternatives are either outdated or cheap alternatives.
 
First thought: don't buy anything without trying the keyboard. For me, the cheap new Windows laptop would be the best choice. Choice of OS would not concern me that much for writing as long as you can get Windows 7.

I might be curious enough to demo an iPad with a bluetooth keyboard and bluetooth mouse or Apple trackpad, but I would be surprised to prefer that set-up for wrting over the larger display and greater choice of peripherals (keyboard, mice etc.) available with a cheap new Windows laptop.
 
I've just finished adding 1200 pages to a multi volume novel using primarily my iPad with a Bluetooth keyboard.

Never before has portable authoring been this easy and this much fun.

I use Storyist and do also tie back to my Mac on occasion, but for portable writing, the iPad is a dream machine for me.

Out of curiosity, how fast do you type on your iPad? I type about 70 wpm normally on a keyboard, but about 30 wpm on an iPad. I suppose if I were a slower typist (or practiced more on the iPad), I would be perfectly happy using the iPad as a primary authoring device.
 
An iPad plus Bluetooth keyboard is a possibility. One advantage of this option is that you can easily use the screen in portrait mode, which gives you much more usable screen space for writing than the pure size would suggest, height being more important than width.
 
If you can afford it, a refurb MBA would be my strong recommendation. If not, an older MacBook - or even PowerBook would probably suit your needs pretty well.

I would not recommend an iPad - not if you plan to do a lot of writing - the touchpad is fine for brief communications and surfing but not for extensive writing.
 
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