Nobody can give you a definitive prediction of the ultimate support-lifespan for either of your candidate devices - but, generally, end-of-life for Update Support appears to when the available device RAM is less than that required by the updated version of iOS.
The minimum RAM configuration for current iPad models is 3GB; the newest 2020 iPad Pro models have 6GB RAM. iPadOS 13.x requires 2GB RAM as a minimum supported configuration.
Of your two candidates, the iPad Pro 10.5” has 4GB RAM, whereas the iPad Air3 has 3GB RAM. Whilst the Air3 is essentially based on the iPad Pro 10.5 platform, the iPad Pro (with A10X CPU) will outperform the Air3 (with an A12 CPU) - and has superior hardware (audio and Camera etc.) to that of the Air3.
From a technical standpoint, if a future major version of iOS/iPadOS were to increase the minimum RAM requirement to 3GB, earlier iPad models with 2GB RAM that are currently able to run iPadOS 13.x would not be eligible for update. It follows therefore that if update eligibility is determined by system RAM (as has seemingly been the case to date), the iPad Pro might have greater “headroom” and potential support lifespan than the Air3.
I must stress that nobody outside of Apple know which iPad models will be supported by the next iPadOS update, and participants of the Apple Support Communities are not permitted to engage in speculation as to future developments.
I hope this information and analysis is helpful in informing your possible decision.
