"Puesta de sol" or "puesta del sol", which is more correct to use for "sunset"? I would think "del sol", since sol is masculine, but I have heard and seen both used frequently.
Both are correct
By principle "del" should be the correct pronunciation, but in Spanish as matter of respect, when talking about specific or unique beings/things, we won't use "del" or "de la", we would just use "de".
De "Person Name"
Why is like this ? Because "del" is the contraction of "de" and "el", and is used to specify something, but if something is unique, then there's no need to specify it.
For my personal use I would use either one depending on the context:
if i'm talking about the sun
you can play until the sun sets
pueden jugar hasta la puesta del sol
if i'm talking about the whole sunset in specific
this sunset was beautiful
esta puesta de sol fue hermosa