Is it better for the battery to put your Mac to sleep while charging or shutting it down ?

It is up to the user.

You can charge the battery when Mac is running, sleeping or when shut down.

Run it on battery whenever necessary and keep it plugged in whenever possible.

When CPU/GPU heavy tasks are run, always plug it in.

 Section: Tips for  MacBooks

 https://www.apple.com/uk/batteries/maximizing-performance/

macOS manages battery health now.

How battery health management helps

The battery health management feature in macOS 10.15.5 

is designed to improve your battery's lifespan by reducing the rate at which it chemically ages.

The feature does this by monitoring your battery's temperature history and its charging patterns.

Based on the measurements that it collects, battery health management may reduce

 your battery's maximum charge when in this mode.

 This happens as needed to ensure that your battery charges to a level that's optimized for your usage

—reducing wear on the battery, and slowing its chemical aging.

 Battery health management also uses the measurements to calculate when your battery needs service.

https://support.apple.com/en-us/HT211094?cid=mc-ols-energy_saver-article_ht211094-macos_ui-04022020