The one you like better. I don't think there's much difference in difficulty. The only language that's considerably easier to learn is Esperanto, but I'm not sure whether your school will accept it.
I'm sorry you don't have any intention of using the language. It sounds like never giving your english-language-brainparts a rest, and voluntarily limit your talk to that which happens to be sayable in one language.
I think that between French and Spanish, French has a lot more similarities to English in terms of the words so I'd say French but I wouldn't teach yourself. Go to evening classes, it's much more enjoyable and interactive and you get to meet new people and hold conversation.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.