Krásný is less strong then nádnerhný. Krásný is nice, lovely or even beautiful, nádherný is more strong word, something like gorgeous.
Personally i would use jak krásné nebe (for nice but ok) X jak nádherná obloha (for amazing), cause obloha is emphasizing also, its bit more poetic.
Personally i would use jak krásné nebe (for nice but ok) X jak nádherná obloha (for amazing), cause obloha is emphasizing also, its bit more poetic.