Neither are terrible but they don’t have the same meaning. The second is vague. Natural are ‘subsistence farming’ and ‘subsistence agriculture’. Examples: They/he engaged in subsistence farming. They/he practiced subsistence farming. Society there advanced beyond subsistence agriculture 3000 years ago.

Number (2) sounds more natural in my opinion. "Subsistence" is a noun that means supporting oneself at the minimum level, so "subsistence existence" sounds a little awkward. "Subsistence" can be used in other contexts and sound natural. Some examples: - "The farmer was a subsistence worker." -> The farmer is self-employed and produces his own food for himself; he is self-sufficient. - "The farmer farmed for subsistence." -> The farmer farmed to support himself/survive.