Dear Simple Living,
Let's take it nice and slow. I want you to have the best experience once you get your new machine, because if it's the best, you will be a Mac user forever and only occasionally venture into the realm of Windows. So, be fore-warned, once you get your machine remember to check it for any issues that may pop up. If there is an issue then take it back and replace it with another one. Not that Apple has wide spread problems, they barely have any over here in Maryland, but I don't want you coming back and posting a thread about how you don't like this and don't like that because of some yellowing screen that suddenly appeared. 🙄 😉
Seriously though, I didn't mean that in any negative way, just letting you know. Now... you're a writer... so the only application that should be in your mind when it comes to word processing (sans Microsoft) is iWork. The application is $80 and you get three apps that run their various flavors of MS Office apps, but do a much better job at handling graphics, audio, video, and anything else besides words that you may put in there. Those apps work flawlessly with iLife and Mac OS X's software, which is what makes the Mac experience so smooth to begin with. There are many free alternatives as well, like the one mentioned above and another called NeoOffice. You can get them along with iWork like many do just in case there is something that doesn't work right with iWork.
I am a desktop lover. I have been using my MBP more than ever though, and may change my game plan. My next desktop will be used for a spare machine to do the heavy lifting, and my MBP will be my main computer. Basically, I say go with the portable. The 15" MBP should be fine only if you don't need the bigger screen. If you want the 24" then get it, and get the 15" later 😉 once you see how lovely it would be to carry Mac OS X with you.
If I could do it all over again, and not have to spend money on the expensive crap just to run Final Cut Pro and Adobe CS3, I would get a 20" iMac and a MacBook for around $3000.