AP Mid Laners, who to buy?

I have now collected 6.3k IP and I'm wondering who to buy. As the title suggests, I pretty much only play AP mid laners. My main is Orianna and I also play Teemo a lot too. Right now, I'm thinking between Azir Syndra or Cassiopeia. My friends and other random people I've ask are pretty much split 50/50 with me either going Azir or Syndra. My play style is poking and winning trades at a distance while cs-ing as much as I can. I don't usually like to engage, especially when the enemy champ gets really close, when I prefer to run unless I'm sure I can kill them.

So, out of these AP mid laners, who do I buy? Or should I go someone else other than the 3 choices I listed? Please share your thoughts with me in the comments below.