Common sense would dictate that, Spanish being closer to French, it may be easier to learn the latter from the former; but I do not think this is entirely true – or at least, that it is not always true. My background is that of a native French speaker who is fluent in English but was never able to master Spanish; I have devoted a lot of time to teaching French to English speakers (whether native or not).
I have to say that knowing Spanish will probably help you a lot with French pronunciation and grammar. Through your knowledge of Spanish, you will have fewer difficulties in understanding issues of gender, "tu/vous" (you), pronominal verbs, and tenses that are little used in English, for instance. However, this does not necessarily mean that you have to learn in Spanish – you know the concepts anyway.
The issue of vocabulary is very important, I think. French and Spanish are both latin languages, meaning that they share common roots, but they have since evolved on their own, and you will find very many false friends and other creeping differences that can actually make things harder for you (some words have nearly opposite meanings, the subjunctive mood is used in completely different contexts, etc.).
English, on the other hand, has practically no latin roots and simply borrowed words from French at a much later period, mainly for use by the court. Being practically absent from vernacular language, these words have been rather well protected and have quite often remained faithful to their French origin. Some words – those which are most used – have evolved and will be considered false friends, and the French counterparts have sometimes evolved too, creating discrepancies. Sometimes the pronunciation and writing of the words have evolved, but usually in predictable ways. Still, most of the time, the English word will be rather accurate or, in the worst case, a bit archaic, but it can still help you grasp the gist of the French word and make it easier to learn.
A DISCLAIMER, however, before everyone starts saying I am wrong and listing the numerous false friends between English and French: I am talking about the extremely numerous English words that few natives know about, that the general public won't understand, and that almost no one ever uses for fear of being told that they write like in the sixteenth-century.
Yet, if you are amongst those who do know these words, they will be of invaluable help when learning French, because then you only need to understand how they evolved (and to re-learn the commonly used words, most of which have become false friends). When teaching French, I usually spend about a third of my time actually teaching English (even to natives).
If you do not have this kind of vocabulary, then I suppose using the language you feel most comfortable with would be the solution.