Your job is not to be an advocate, it's to support your users to the best of your ability. That means steering them towards stuff you know how to support, but doesn't mean pushing them towards your favorite hobby OS when that's not a good match for them.
The most important thing is to support what your users need to do. For creative users, including writers, that means the tool they're familiar with. If they're used to Windows and Windows tools, give them Windows. If they're used to OS X, give 'em a Mac. Either way, give them a usable screen and a good keyboard - writers will likely kill you if their main writing machine has a plasticky 93% size keyboard.
The second most important thing is to make sure that the systems are available and the data is safe. Which makes Windows a PITA to support if you're not familiar with Windows administration (which is what it sounds like).
If it were me, I'd use Macbook Pros running OS X, with VMWare Fusion with unity mode turned on allowing me to run Windows (or Linux, come to that) applications, as though they were native apps. (Don't skimp on RAM). That way the machines can be shared by users who prefer different apps to do what they do, and you can take advantage of either the OS X level stuff or the underlying unix to do backups.
And a couple of cheap netbooks for emergencies, email, throwing in the back of a truck, that sort of thing. Then a bunch of robust, cheap media for ad-hoc backups (USB sticks, CD-Rs).
But I'm not your users. Ask them what apps they need, then work out how best to support them.