Is Germany and Slovenia in terms of Parks and Nature?

They are very different. In Slovenia you will have spectacular mountains, lakes and rivers. Check out the Postojna caves if you have time. The Soca River is gorgeous.

Agree. That's a bit like comparing the Appalachians to the Rockies.

Both are amazing, but VERY different.

Germany is more varied--it has lowlands and marshes in the north ("Norddeutsches Tiefland" or North German Lowlands), "mountain" ranges of medium size, including the Black Forest ("Mittelgebirge"), the Alpine Foreland or Bavarian Plateau ("Aplenvorland") and in the extreme south, the Alps.

Slovenia is far more Alpine and far wilder. In Germany, every corner is managed, and you have wonderful nature--but you don't have true wilderness. In Slovenia, you do. Slovenia is likely to have far steeper hikes with gorges and such, while the Black Forest will be, as the name implies, densely forested.

You can get amazing hiking and nature in both. Both are clean, safe places where people speak English, and both have cute villages. You will have an amazing time in both, but you won't be getting the same experience in both.

As HowlinMad said, there is a great diversity of landscapes in both countries.

To be sure, there are also steep, rugged mountains with gorges, rivers and lakes in Germany. Search for Berchdesgaden, Mittenwald and Garmisch Partenkirchen.

On the other hand, Slovenia is not only made up of the Julian Alps. 60% of Slovenia is hilly forested area. Especially in the south, in the dense forests of the Notranjska and Kočevskor region lives one of the largest populations of brown bears in Central Europe.

The Black Forest and Slovenia are not easy to combine in one trip. You could check the night train Stuttgart to Ljubljana or fly from Zurich.