Ahri's great. Her and Azir are my two midlane mains, I usually play her when Azir gets picked or banned away from me.
You can max either W or E second on her, it really doesn't matter too much. Technically if you're hitting your charms then E max will do more damage, but W provides more consistent damage especially if the enemy team has melee champions that like to run at you.
W also is more dangerous damage because the range is pretty short compared to the rest of your kit. In a situation where you're trying to burst someone down, E max is better. In a situation where you're trying to chase down or run from a tankier target with your ult, W max is better. Just base it on the situations you think you'll be in.