9a has bigger bezels, a slightly worse screen brightness wise, and notably worse cameras than the 9. That is alongside slightly slower charging speeds and less RAM. If you don't care about the camera quality difference, mainly noticeable in low-light conditions, or the decrease in RAM (usually not noticeable anyways), the 9a is usually the better buy.
The only thing I will note is that the a-series for Pixels historically have had battery issues. The 4a had massive battery degradation issues, the 6a battery swelling and a whole replacement program. Some 7a phones also had battery issues. Nothing yet on the 8a or 9a, but it might be too soon to tell if Google has actually fixed the issues now.