@drummer 

I'd Google your question, there are A LOT of links on this.

For instance:

This one link is very good, https://versus.com/en/tp-link-archer-a8-vs-tp-link-archer-a9 and the major differences can easily be seen. Major seems to be for the A9 is Has WPA2 Enterprise, which is NOT usually needed at home.

This one, https://www.bhphotovideo.com/c/compare/TP-Link_A9_AC1900_vs_TP-Link_A8/BHitems/1468480-REG_1601816-REG, would have me select the A9 as it has USB support which to me implies DLNA support which I use.

This was also asked here 2 months ago, https://community.tp-link.com/us/home/forum/topic/246580 (however today I think the prices are the same).

Still, you have to make the decision.