さ・み: the difference
さN4
Meaning
Converting adjectives to nouns
Past papers
Example
こんな美しさは初めてです。
I've never seen such beauty.
みN3
Meaning
~ness, Used to indicate conditions or nature
Past papers
Example
このスープは魚のうま味が感じられるね。
I can taste the deliciousness of the fish in this soup, right?
How to tell them apart
「さ」 gives a measurable degree and works with almost any adjective.
「み」 gives a felt quality or a part that has the quality: 「悲しみ」 (sorrow), 「深み」 (depth, deep part), 「甘み」 (sweetness). Only a few adjectives take 「み」.