I have a friend who does serious writing (screenplays/scripts) on his iPad to prove a point. But we all know he sneaks off to his MacBook Pro for 99% of his actual writing.
As a scribbler myself I'd go MBA. Actually, if writing was my main activity I'd go cheap and cheerful lightweight PC with Word or Open Office. I like Pages, but it's a pain if you need to send documents and manuscripts out to the world (the conversion-to-Word feature always messes up the layout).
I got forced onto a PC these past few weeks after my MBP went on the fritz. I'm a long term (20+ years) dyed in the wool Mac user and after a few initial days of frustration I'm actually starting to enjoy, and even prefer, Windoze 7. Weird.