whats better 2021 macbook pro or 2024 macbook air

When you say "2021 MacBook Pro," I assume you're referring to the 14" and 16" MacBook Pros with M1 Pro and M1 Max chips that Apple released in 2021. Not to the 13" MacBook Pro with a base M1 chip that came out in 2020.

14" and 16" MacBook Pros with 'Pro' and 'Max' chips have some things that MacBook Airs with 'base' Apple Silicon chips do not:

• Mini-LED backlit screens
• A third USB-C (Thunderbolt) port
• A HDMI port
• A SDXC card slot
• Support for two (Pro chip) or more (Max chip) external displays with the lid open
• Often – more CPU cores, more GPU cores, and more RAM than Macs using 'base' chips of the same generation

On the other hand, you're comparing M3 MacBook Airs to M1 {Pro/Max} MacBook Pros. Given that the MacBook Airs in question are newer, you'll be able to get hardware repair service (should you need it) for them for longer. Also, it's unlikely that anybody has many new M1 {Pro/Max} MBPs left in stock, so unless you're looking at Apple Certified Refurbished units, there would be the question of a used machine's battery health and overall condition.

Finally, if you are considering buying a used Mac, you might want to read

The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community

That article was written about iPhones and iPads, but some of the same cautions would apply to purchasing used Apple Silicon Macs.