Update doc vs create new doc for small nested data

[sccds (Xiao Liu)]

Hi, I meet a question for update doc vs new doc for nested data.
My data is like:
{
"some key": xxx,
"key": [
{"nested key1": value1},
{"nested key2": value2}
]
}

In the nested key part, about num 3-5 data integrated in one json file in 10s.

I can keep append data(update) into the same _index/_type/_doc, or create a new doc with small new data. Which one is better for this case?

What is the default index refreshing in ES?

Which one is better for space saving? how is the "update/create" performance? how is the searching query performance?

Thanks!