According to me all the history subjects are good and it really depends on your background and interest. To be honest, i would suggest to keep three things in mind for picking any of the history subject.
1. Your Background
2. Resources and Time available
3. Your interest

If you have a background in any of the history subjects then go for it. To be honest, there is no such thing as scoring or less scoring. It depends on the individual. I went for European History because I did that in A levels. It is definitely longer than US history, but it covers the paper 2 of IR entirely. I had interest in EH as well because if you think of it, it covers a lot of historical events related to nationalism, war, peace, democracy and etc. All of this can be discussed in the essay.

Now coming to your question, US history has many merits, like it is short and there are multiple online resources available. So i would suggest US history, but you will have to put an effort to make your answers better.

I can suggest the following:
They want your analysis of the events and not the history of the events
Analysis is breaking the events down into factors and different aspects.
Most of the US history is a white Christian Male's perspective, so your analysis must include some of the African American and Native American perspectives. This will add different levels to your analysis
History is about the values of people of a time. You should be able to connect to those sentiments and values. You can't understand slavery unless you understand how African Americans people feel about it.
Another thing that will make your answers really good is evaluation of events. Now evaluation is different than analysis, this is your critical assessment. For instance they ask about some treaty (i haven't studied US history so I can not point out exact topics) and its success.
Your critical assessment can be comparing with with another treaty that was more or less successful. The question would not directly ask for this, but adding some evaluation will show the examiner that you have a critical mind and you can connect different events. Now don't make your entire question about evaluation. In fact follow this
20% knowledge (what was the event)
50% analysis (breakdown those events and add different perspective)
30% evaluation (talk about success and failure of those events and compare them with a similar events)

History papers don't test your knowledge about history, but your understanding on them

Hope this helps!

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