it doesn't really matter that much since they would be able to understand you regardless of which one you learn. I think the bigger question would be who do you think your more likely to speak with. If you think your going to be working with american's then learn the more american phrases, if you think you'll be talking with a lot of brits learn british english.

I'm a native to the US, I'd say British English is the best to learn. American and Canadian tends not to be as proper (especially the Americans). I'm not sure about Australian or New Zealand, but the Brits usually speak fantastic English. Hope this helps.

If I were you, I would learn Aussie English! :-))

Hey -- yeah I agree with Tofu. Learn the accent that you think you'll be using the most. I'm American, and I lived in the UK for three years. There are many British accents -- some are great (London, upper class, etc.), and some are not quite so standard. But I think this is more of a question at higher levels. If you can acquire an American or British accent, as opposed to a foreign accent, you're doing pretty well.