I like Ahri a lot, probably one of if not my favorite champ but Diana kinda dumpsters everyone. As much as the boards may want her nerfed for whatever reason, Ahri kinda struggles in a few matchups and can be bullied easily where Diana will typically win most lanes especially post 6. Diana can also be a very strong jungler and she has a crazy amount of solo carry potential.
Diana burst is kinda silly, squishies will be deleted most of the time even without her reset if theyre not 100% hp. Ahri is safe, her mobility allows you to run away or burst someone down and escape, the movespeed on her q allows you to poke in lane pretty safely and dodge return harrass. She's good at all stages of the game and will be generally useful even when behind whereas Diana burst will be less impactful from behind because she doesnt really have CC, just the ability to nuke someone.
Diana is VERY strong atm, I'd argue were she more popular people would want her nerfed like they want Ahri nerfed so I would say probably Diana better to get but as an Ahri fan, can't go wrong there either.