A good rule of thumb is that early levels are easy for ranged ap casters and late levels are more difficult (vice versa for melee champs). There are no set rules to the game though. Each game can be unique in its own way. If you choose a melee such as Diana then the laning phase will be more difficult then say Morgana or Ori. However, usually melee champs post 6 can burst down enemy champs much easier than there ranged counterparts.
Estimate what champs will be best for you by asking yourself, "What do I like to do in-game; what is my play style? Am I a safe player or ballsy? Do I like to roam?" These questions will help guide you in your search for a mid-laner that fits your specific play style. I really like Morgana because she is extremely safe in lane; it can be very hard to gank her. Plus, her cs-ing ability, cc, and support capabilities are excellent. She can be mana-hungry at early levels and might need a blue to stay in the game depending on who she is laning against.
Hope this helps :)