League of Legends (LoL) Question: Which champion is better to main akali or diana? (Mid)

Diana is way better. Akali is indeed strong, but Diana excels in midlane as a tanky assassin. Here's why:

While Akali is a bit item dependent during the early game to start snowballing (Hextec Gunblade), Diana only depends on level-6, what means that you can hit really hard once you get it. Plus, being a tankier assassin, her all-ins are better, since it takes a bit more time to kill Diana, few seconds, but seconds are enough time to kill another assassin or ADC. As well as Diana's Moonpale is such an interesnt defensive and offensive abily that really helps you to directly trade with your opponent, instead of going stealth, but still being vulnerable to smarter enemies.

I think Akali is better, coz of the dmg, burst and she is not hard ;D

To me Akali, since when she hits that lvl 6 buff she is unkilable

Personally, I'd say Diana wins. While Akali has a lot of burst potential, I believe Diana can go just as much, if not more. And even with less gold. Diana has a higher skill cap than Akali (once again, personal opinion) and learning her is more valuable due to the fact she is also viable in the top lane, and in jungle.

The only direct difference I see from playing Akali would be the use of her W, Twilight Shroud. Diana and Akali suffer from all-in syndrome, they have no "real" way of getting out other than flash. Diana's E, Moonfall, is a strong AoE soft CC ability that I think out-preforms Twilight Shroud, outside of the fact it stealths Akali. I also believe Diana has more options in an engagement due to her more versatile kit.

tl;dr: I'd personally say Diana.