I'm currently level 21 and I've just gotten another 3150 IP.
I'm interested on buying a 3150 IP champ but am unsure on who to get :S
I want a champion that is good in ranked, when I get there in the future.
So far I'm tossing up between Olaf and Heimerdinger.

Please post your opinions on the listed champs or post ones I haven't listed, but please tell me why you think they are good, not just a name.

Thanks

Nidalee is probably the best bang for the buck imo. She has lots of room to grow as you grow in skill.

Akali isnt used much in rank but shes fun to play

Heim is just hard to play and his damage in underwhelming.

Olaf. Is really good.
You can carry pretty hard, and when he gets fed he can go rambo.

IMO, Katarina, Sona, Karthus, Corki, Shen (if can sneak through bans), Blitzcrank (same as Shen).

There is nothing special about a 3150 price point. They arent any better than the 1350 and 450 ones. I would make sure I checked out all the cheaper ones first and used those to form much of my champ stable.
At your level, you hit the dreaded Rune Hell where you go from level 1 runes to tier 3 ones. This costs a fortune in IP but will make you more competitive having twice the rune strength as all the other 20ish level people.

Sona is alright, Though I'm fond of support.

Xin is alright.

Out of the 3150 champs, not many stick out for ranked matches. Anyone but Karma or Shen will rare play in ranked (outside of Blitz/Sona). Shen is forever banned and Karma is a champion noone plays ever.

If you just hit 20, your IP will be better spent on runes. May I suggest tier 3 Flat Cooldown Glyphs?

I play Olaf and he's a great anticarry. Two of his abilities scale better building health, and his passive attack speed, bonus lifesteal on his W, and Armor and Resist on his Ult make him dangerous in a 1v1. His Ult also stops all incoming disables, so Ghost is a good SS that lets him get to the AD Carry and melt them. Fastest jungler in Season 3.

Heimerdinger... He's good on Dominion, Zyra is better at the tower defense game in SR.

Between the two I would definitely go for Olaf over Heim if you really want to bring a char into ranked. A properly played Olaf is a strong asset to any team and is very hard to kill.

You're level 21? Have you spent ip on runes yet? If not I would strongly recommend building a few strong rune pages before getting any more champions.

Of the two champions there, I would say Olaf. He is an S-tier top lane and can be good in the jungle too. Brings a lot to the table and can peel like nobody's business