From what you wrote, Sour Lemon, you're doing much better with English than some native English speakers do. You might try posting a short snippet (~250-500 words) in one of the Share Your Work Forums and ask people there how you're doing with English. If you do that, please post here to let us know where in share your work you posted.
Another consideration is what you'll be doing with your writing. If you hope to publish in the US, then writing in English is better; if you hope to publish in a Spanish speaking country - you know the answer. Good luck! Puma