Need Help Choosing New Champ!!!

I've been putting it to thought recently, Zed has always been a good assassin, but Rengar looks like he could be a cool champ to play as too. I know Zed takes skill but I never played as either of the champs. I'm in to assassin types so they both kinda fit my list. Most of my friends have been saying it's how you play, but I'm asking for OTHER PEOPLE'S OPINION. Not mine, so if anyone can help me make my mind please do. I know I can always refund but I don't know how to. So if you can also tell me how to refund that would be great!