They are not the same - they are two completely different web sites. Just because they are both hosted on the same server doesn’t make them the same. There isn’t even anything to show that they are both owned by the same person/company.
Why wouldn’t they combine them together into one. I don’t get it. It makes it confusing for people to pick which one to sign up for if they don’t know what’s different about each of them.
It looks to me as though both services are run by 1&1. They would be the most obvious people to ask what the difference is between their 2 mail offerings.
If you don’t want to do your own research by signing up to both, you could always try searching the Internet for information. (Novel idea, I know. )
From that second link:
In September 2010, Mail.com Media sold Mail.com to United Internet, which intended to integrate it into GMX Mail. While existing accounts could be accessed from anywhere, users accessing the site from German-speaking countries could no longer sign up and were instead invited to use United Internet’s GMX services geared to those markets (gmx.de, gmx.at, gmx.ch).