It depends really Anivia has a lot more survivability but harassing is harder till 6 where as cassio can harass from lvl 1 but survival is harder and she is more suceptable to ganks. Over all I prefer cassio over Anivia because I like a agressive early lane.

I ALWAYS play as cass... I love her... If you wanna get alot of kills and bait the eenemy into thinking they could kill you... play cass

Even though I'm a pretty bad Anivia player, I'd still have to choose her over Cass. She just brings more to the table with her wall/ult. A great Anivia player > An excellent Cass player.