Okay as someone currently on a year abroad in Germany, this is important, unless you're going somewhere with a high percentage of English speaking students, or is set up to take in lots of English speaking students, it's really best to go to a country where you have strong language skills.
It's just that much harder to make friends if you're struggling to quickly speak in a conversation. And German isn't an easy language to learn, although a lot of the vocabulary is very similar to English.
I wouldn't say that Germany is the most dissimilar from the US either, I've never been to the USA but Germany is probably easily the country in Europe most similar to Britain. They've got our drinking culture and we're fairly similar people with similar levels of openness (although thats definitely not true of the US and Britain =D), whereas Spanish people genuinely behave a little differently.
Also German food is boring. It's meat and salad (often weird salads with Gherkins). Always. But absolutely everyone is fluent in English (including a homeless person, who had a conversation with me in perfect English)
The only factor I know about Spain is that the stories of it's economy being bad right now really aren't exaggerated. People are struggling to find jobs and everything is being cut to the bone. In some ways this can be a good thing, because everything is becoming very cheap, my friend on a year abroad in Spain said you can basically just turn up and find a house within a week (although halls are better because you have more people around you)
Other than that, have fun, it can be an absolutely brilliant experience and many other people who've went away have wanted to come back. But consider everything carefully because it's not impossible to have a less happy time. There's a lot to pick up in a very short time period and a lot of things will be harder than they normally are. If you happen to be a fairly shy person, I really would recommend an English speaking country, but otherwise I hope you have a fantastic time wherever you go.