It depends on your playstyle really. As I said before, you should just try all the Free to Play champs and buy something you like from that group. That way you won't have to sink gold into a champion you might end up not liking. To someone who has a large champion pool, this doesn't matter, but to you, who is still low level with a smaller pool, it can turn you off to the game if you make a poor purchase(like not liking the champion).
The only mid laner that I see at 3150 that is worth buying is LeBlanc, she is pretty OP. She is an assassin that blows people up. She can win lane hard if you play it right and snowball out of control if you get kills. Her main flaw is the same as other assassins. If she doesn't get gold from either CS or kills, she won't really have the power to execute her opponents. Also assassins fall off somewhat later in the game because people start getting tanky enough to withstand her spell rotation.
I guess if I had to mention more, I would say Katarina or Urgot. Katarina stomps hard on enemy mistakes, but she is shut down HARD by stuns/silences. She has decent mobility and is a reset assassin, so once she gets a kill she can wipe the enemy team in the right scenario. I'm only mentioning Urgot because he is LCS picked. I personally don't like the champion because he needs to be played really well to be able to pull him off, though if he is your thing you can try it. He is a lane dominant champion that can completely bully his opponent out of lane, shutting them down. He has a suppress with his R and shuts down assassins pretty hard. Also his build path makes him get beefy and tanky, so he won't die easy. His flaw is that he needs to win games early, as his damage isn't as good as other champions in mid lane. This is harder to pull off with randoms imo, which is why I personally don't recommend him.