My sample is not in any way scientific so take the results with a grain of salt.
A few sentiments came up repeatedly: people say AT-AT but spell out AT-ST, the people in the spelling it out camp seemed much more likely to say "my way is the right way and everyone who doesn't spell it out is dumb" (I'm paraphrasing a little), and only a few people were in the Imperial Walker camp. And a couple of people pointed out that TIE Fighter is rarely spelled out, only said. I happen to like the way AT-AT rolls off the tongue, so I'll stick with that pronunciation. I want to get the @ @ t-shirt.
What do you call an Imperial Walker?