in Genitive Singular for Masculine and Neutral nouns, it's added "-s" or "-es"? [duplicate]

Question

In Singular Genitiv for the neutral noun "Das Buch" they wrote in Duden "des Buches, Buchs", is there a rule to choose if we must add "-s" or "-es" to the noun, or something else?!!

Answer 1

The answer, as often with languages, is: It depends. The question itself however needs to be tackled in two steps:

Determine allowed Suffixes

At first you need to make sure which genitive singular suffixes are actually allowed for a given word. There are three types of nouns in the German language when it comes to this:

• -es only, clearly marking the case suffix:
  • all nouns ending with s, ß, x or z, e.g. des Glases, des Straußes, des Reflexes, des Sturzes
• -s only, for more melodic intonation of the word (-es would sound wrong) or alignment with the plural:
  • many nouns ending with an unstressed syllable, e.g. des Abends, des Reichtums
  • all nouns ending with an unstressed -e, -el, -er, -en, -chen, -lein and ling, e.g. des Vogels, des Lehrlings, des Bodens
  • most loan words, especially those with plural-s, e.g. des Textils, des Generals, des Hotels
• -s or -es:
  • all others

The inflection class of a word is also an indicator, whether only one option is allowed.

canoonet gives a more detailled overview with many more examples.

Choosing -s or -es if both are allowed

For a word where both endings are allowed -es is indeed considered more poetic as stated by accolade, but could also be interpreted as outdated. The Duden says that -es is preferred in constellations where the genetive is prefixed (e.g. des Tages Hitze) or within compounds with interfix-s (des Geschäftsfreundes).

Answer 2

Folgendes stellt keine feste Regel dar, aber ich würde sagen, -es ist lyrischer/ (gehobener(?));
-s ist der Normalfall.