AmphibiousExploringVehicle do note that there is also differences in modems, too. not just applications run better, the Pixel 9 series excluding 9a/10a with the Tensor G4 would be vastly superior than a Pixel 8a in cellular connectivity too.
A search for "Pixel 8a modem" on DDG even showed me two Reddit (reddit.com) top results like Returning my Pixel 8a due to modem issues and Pixel 8a modem issues? This means any modem complaints for 8a, if a dealbreaker for you, would mean 9a should not be used (since it shares the same modem as the Pixel 8 series, source below).
The 10a has an Exynos 5400 modem (androidpolice.com) instead of the 9a and the 8 Exynos 5300 (androidpolice.com).
The 9a and 10a both has superior battery capacity than the 8a, and the new CPU (Tensor G4) has both a fair gain in power efficiency and total performance, and less charging will reduce the number of total charge cycles, allowing you to keep your Pixel 9a/10a batteries not repaired since purchase for a longer period of time.
If you want to maximize your value, a 9a is a good choice, but only if you can find one (I would rather not buy an old phone due to poorer water and dust resistance due to natural weakening of the seals, but maybe you would find it fine.)
The 10a has the benefit of being one year newer than the 9a and is still in stock despite being almost the same as in spec on paper, and maybe if you can hold out to Black Friday/Cyber Monday you can catch some deals. I would buy a 10a in cash strapped situations because the total long term cost could be less, and in the future the value you might get from reselling a 10a would be slightly higher.