Hi adeb97,
Putting your MacBook Air to sleep lets you quickly get back to what you were doing when you open the lid or wake it up. Everything remains exactly as it was when you last used your Mac. It is the most convenient option of the two.
Shutting down your Mac closes all open apps and files, and then powers off your Mac (except for the Always On Processor, which detects things like opening the lid, pressing the trackpad or keys, etc.) This is the most secure option, as several Data Protection keys are completely locked until you power on and log in again. For the best security, enable FileVault so that your password is required to access your data, and all Data Protection keys are locked out until you log in again.
In terms of the overall health, battery, and SoC, it's best to use your Mac as you find most convenient. macOS and your Mac's hardware intelligently manage power consumption and battery usage. That said, to reduce the amount of battery usage, keep your Mac plugged in to power wherever possible.