Lat Lon are always decimal and not string as you can see from several links:
- In Blog: Easily build location-aware apps
- In Documentation: Representing geolocation data
- In Discuss: RFC for implementing GEO features in GraphQL
And decimal is universally accepted:
- How many decimal digits to store Lat Lon?
- GeoJSON format also stores location data in decimal.
- Google Maps uses decimal as well.
TL;DR: Store the Lat Lon in Decimal format, so that both client & server can prevent unnecessary type casting.