I'm going to have to concur with Abba San here. If you HAVE a desktop/machine at the house/office already, go with a tablet. And, again, I'd choose the Xoom (or maybe the Transformer).
In fact, I'm currently looking at those two devices as possible tablet purchases for me an the Mrs. In fact, the wifey and I have been discussing the possibilities of forgoing replacing her laptop when it inevitably dies here soon, and taking her to only a tablet, with the ability to use my desktop for those very few tasks that she performs that are not tablet capable.
That digression aside, there's another note to take consider while we're talking about devices for preaching (or any form of speechifying), I have found a Kindle (sans cover) to be a good sized device to hold the necessary notes for a speaking role. Of course, when I'm in that particular role, I will have my speech more or less memorized, and will just take bulletted notes to keep myself on path and as memory hooks. The Kindle performs that role nicely without being something that gets in the way of your speaking. Additionally, you won't have to change device usage patterns if you're having to speak outside. It's also nice not to have to shuffle note-cards or pieces of papers as the talking points progress.