Iphone Game development (later on Android, too)

Hello, im looking professionals who can development my iphone game idea. Its a little shooting game, where you can shoot people on top of a skyscraper. And you shoot down on people. More details in the briefing then. The app should be high quality, very smooth and be able to convert easily to android later. The app should have in app purchases. The app should have a beginning screen where a little popup comes up, what i can manage over a webadmin. (to promote some images, and text on it) When you bid, please send me some experiences you did before, and maybe you made some graphics for this, too, so show me this, too, so that i can choose the right person for this. So i know titanium, but i dont know whats the best solutions for games. So please provide me with your prefering... thanks