I think you should get Ravensword... Or if you have UPNP on your router, get Modern Combat, its the best engine on the iPhone
red conquest just came out, best rts on idevice http://itunes.apple.com/us/app/red-conquest/id342960702?mt=8
I find ravensword even becomes laggy on iPhone 3GS sometimes so not sure if it will go well on other devices...
Personally I feel you could go for 10 $0.99 games, or get 1 $9.99game, or get 2 $4.99 games. I preferr to get moden combat and blades of fury.