Magsafe 15W or Apple Wired Charger 20W

If you're asking "which method is better for the health of the battery?", I don't know the answer.

Otherwise, it appears to be just a matter of convenience (not having to plug and unplug cables) vs. charging time.

Apple says that "If you have a 20W USB-C power adapter or higher-wattage USB-C power adapter — like the one that came with your Mac laptop — you can use it with your iPhone for faster charging."

The MagSafe Charger is compatible with the iPhone 15 Pro Max, but only supports charging at "up to 15W" – i.e., not at high enough levels to "fast charge" an iPhone 15. The MagSafe Charger itself is just a charging pad with an attached cable. You need to use it in connection with a separate USB-C power source, like the Apple 20W USB-C Power Adapter.

Charge and connect with the USB-C connector on your iPhone 15 - Apple Support

MagSafe Charger - Apple

Buy 20W USB-C Power Adapter - Apple

there is no best, your battery will begin to decline no matter what you can only delay the process, the wired chargers charge faster more use less current pr. batt % though

have you read

About the battery and performance of iPhone 11 and

later - Apple Support

and maybe

iPhone Battery and Performance - Apple Support

and Batteries - Maximizing Performance - Apple

people report apple does not change the batteries until they are lower than 80% in health