Google search algorithms are based on how we humans understand things.
The most important thing is being consistent with usage. Don't give Google confusing signals.
My advice for usage would be this...
St or St. are also abbreviations for "street" in the English speaking world.
I would use "Saint" .
The flip side of the coin....
Google is pretty accurate sometimes, actually mostly correct. So if you write excellent content, Google will know exactly what you meant with your use of St, St. or Saint. The content on the page will provide the necessary hints for Google to know your usage of the term.
It would also depend on the competition in that vertical niche term:
- Which term has the potential to bring in more traffic.
- Which term is the most competitive.
- Which one's are low hanging fruit.
Go for the low hanging fruit to build traction in the market then work on the harder more competitive terms.