Sumerian script was older and deciphered with its own three language key tablet.
The hieroglyph and cuneiform scripts are about the same age, and were in use for a similar length of time.
Sumerian and Akkadian and a number of other languages were written using cuneiform, and the older Sumerian system was cracked using Akkadian, an extinct Semitic language, which made it possible to pronounce the symbols, and by the 20th c, the language isolate, Sumerian, was read for the first time in thousands of years. The writing system was versatile and in use for over 3000 years.
https://en.wikipedia.org/wiki/Cuneiform#Decipherment
https://en.wikipedia.org/wiki/Akkadian_language#Decipherment
https://en.wikipedia.org/wiki/Sumerian_language#Historiography
“he hieroglyph and cuneiform scripts are about the same age”
I think there is a dispute among experts as to which is older.
In any case the Sumerian language was a phonetic invention which to my mind is more complex than a language developed from pictorgraphics. The sumerian language also bred literature and was used by more cultures so in that sense it was a more important development.,