It might be less obvious than it seems. You certainly can use both languages to learn German. And basically, they would complement each other. While English has a very nice and logical system of verbal tenses, the rest is more or less a mess... which means that you just have to learn most things by heart. This is very much unlike German, where mastering the rules will save you A LOT of work if you just use your brain. Here the much more logical structures (in particular: word formation) of Spanish will come in handy. Also consider that in spoken German you can get along quite elegantly with two and a half tenses (Präsens for present and future, and Perfekt for the past, with a handful i.e. 5 or 6 cases where Präteritum is adequate). English might be somewhat helpful on a vocabulary level (40% of English words are of Germanic origin) though. On a practical level, both English and Spanish are OK as medium of instruction. Still, you will certainly find many more German teachers speaking English than Spanish, so that will give you more choice. I hope this helps.

Hola, Alejandro. Me parece la tuya una duda muy razonable y da que pensar. Sin embargo, en vista de que tu inglés es de un nivel nativo, me decanto claramente por este idioma ya que si bien el alemán tiene muchas particularidades y es bastante más enrevesado, si que es verdad que ambos están emparentados y algunos conceptos te parecerán similares. Un saludo a Puerto Rico!

Mi disculpe!Es el mi celular que es roto...

Inglés! La estructura inglesa es más parecida a la alemana.

Inglés!