Having been a student of German language and culture for almost 10 years and an employee of the German government for almost 1 year, I would say that German is the harder language to learn.
Which is more valuble? If you plan on doing business in the States, especially in CA, TX, or FL, Spanish is a must. If you plan on working in the automotive or chemical industries, I'd say learning German is a good idea. In fact, part of what my office does is go to companies like Porsche NA to teach their employees German.
Either way, learning a foreign language is a good move. It expands your understanding of the world around you. Pick one and stick with it!