ARS Technica takes a look at European privacy laws and concludes that your e-mail is probably safer in the US. For instance, one of the things the site unearthed is that in Germany, firms aren't evened allowed to say anything if they have to hand data over to authorities. Check out the full article for an analysis of how secure your e-mail is.
In America, the targets of criminal search warrants almost always don't know those warrants are coming, as they're typically sealed. The United States also has National Security Letters, which prevent recipient companies from speaking about searches publicly. And as the Foreign Intelligence Surveillance Court has come under greater scrutiny, it's become more common knowledge that its orders are sealed as well.
So while Germany may not have secret courts, its e-mail services still have to adhere to court orders that cannot be disclosed to its targets. However, an American provider could notify its customer that he or she is the target of a judicial investigation. Google has a user notification policy, for instance, that stands unless the court forbids it from disclosing that information. (But Google also admits that it is tracking much more information about you!) German court orders, by contrast, appear to be sealed automatically.