I think that few, if anyone, could really answer this with certainty. I think that you have to take some factors into account. The first would be what evidence or analysis you have. The more you have on one particular side might be more persuasive to you in terms of writing. For example, if you can collect more information on one side over the other, I would think that this might help make the decision as to which topic on which to write, if all things were considered equal. Another element would be which one you find interesting. Perhaps, this might be the most important factor. If you are not interested in a particular topic, writing a paper on it will be painful to compose and more challenging to read and assess. Which topic do you find more fascinating? This might be one of the most basic of questions, but could be seen as vitally important in terms of the composition of your paper. For example, could you find a modern connection between assessing the factors that led to the rise of the Nazis between then and now? This might help to foster an interesting conclusion at the end of this paper. Could you find some modern connections between the causes of World War II and today? This, too, might help to develop and interesting close to the paper. I think that being able to assess which paper has more evidence and analysis and which one gauges your interest more might be critical elements to take into account before composing your work.
What were the factors in the Nazi rise to power is a better choice, because it is more specific. There are many events that led to the Nazi rise of power, but also many that led to World War II. You could write an entire paper on a single factor of either, however, I know for a fact that there is enough material to write and clearly elaborate on the Nazi rise to power. I wrote my senior thesis paper in college over a very similar topic. At first I started with a broader topic such as, the causes of World War II, I soon realized in order to fully elaborate without writing an entire book I needed to refine my topic. The key is to select a topic broad enough to have an adequate amount of material, but limited enough to be able to cover the material thoroughly without becoming excessive or repetitive. There are clear and distinct reasons for the Nazi rise to power and that topic covers much less than the second option. A 1500 word essay simply cannot cover the causes of World War II with enough detail and accuracy. Remember all the information in the world is of little use if it is not accurate. Whichever topic you choose be specific and use facts.