On the one hand, writing in English is better because the phrases and sentence structures you will end up using will sound more natural to the reader. On the other hand, if you are writing the first draft before having a crystal clear idea how your argumentation will flow (i.e. if, for you, writing is also a tool for thinking), then writing in English might take take away from this process because it's difficult.
I would certainly agree with everyone else that using English for the first draft is a good idea. But when I write, I also write an outline of the draft in a separate document. This outline summarizes each paragraph I plan to write, usually in a single sentence, so that it gives me a good overview of the structure the manuscript is going to have. I use it to combine the points I want to make with other findings I think are important to mention (usually I find that these don't match up very well on the first try!), and then I shuffle things around, add and remove items, until I have a story that will flow naturally from the questions I ask to the conclusions I make. And only then do I start writing the draft. And this outline, I would recommend writing in your native tongue. It should be easier to switch to English afterwards, when you begin writing the draft, because you'll already have a clear idea of what you want to say.