I'd say the worst thing about the test is the nerves (because you hear from everyone else how hard the test is). If you are more confident in an automatic and just want to drive that, it should be much easier if that is what you are already used to.
I took 20 hours of lessons from a really good teacher (it was hard to find an automatic teacher and he normally taught other driving teachers how to pass their tests, so he was extremely competent). I probably would have been ready after about 14 hours of lessons. I found the test really nerve-wracking, but straight-forward.
Disclaimer: I tested on manual because I didn't want to be restricted to more expensive/harder-to-find automatics when I finally get a car and because I preferred manual in the US. But I recommend you test on whatever you're most comfortable with. Don't add the stress of trying to learn something new to trying to unlearn your American style of driving.
Having said that, I agree with physicskate. The worst thing about the test is the nerves because of all the hype about how hard it is. It isn't
hard. You just have to pay more attention, and adhere to a few more little persnickety practices.
I drove for 20+ years in the US before I moved to the UK, so I had a lot of US habits to unlearn. And after my first UK lesson, I was really annoyed at how critical my instructor was over what I thought were really little things. But it's only because the examiner really does look at all those little things. You really have to drive as if you are a robot, doing everything exactly by the book. Pay attention to everything you're doing. (It's only for less than an hour, and I have ADD and got through it.)
I had six hours of lessons in three 2-hour slots. The first two lessons, about a week apart, were a month before I had my test. The final lesson was the day before my test, because my instructor likes her students to have one last "don't forget to..." lesson just to be sure they are really ready.
Anyway, she predicted I would probably have around 5 minor faults, and she even predicted what they would likely be. She was right. But I only had 4, and two of them were for the same thing that I did twice (more than 15 minors and/or a single major means you fail).