When I write in Spanish I feel myself moving through a medium with a certain texture, one that offers me a certain resistance. I can savor the language itself. In English, I have a much smaller sense of the language as language--at least when I write prose. I don't have a sense of the distinctive Englishness of English. I'm not saying English in inherently more transparent, just that my own relationship to it is colored by the fact that it is my native language. I don't have that inherent enjoyment of the English language itself, sad to say.
This suggests that I should write more in Spanish, the more pleasurable language. There is also a different relationship to the self, for me, in a second language. The speaking subject is not me, and that is part of the pleasure. I'm sure I would enjoy writing in French too, if I weren't so afraid of making grammatical errors.