I don't own an Apple device, so I can't comment from personal experience, but on various forums (including the Slim Devices one) users of iPeng are very happy with it, and many prefer it to the controller.
The controller does do one thing the iPeng device can't: setup the Duet system from scratch. I know this because my controller bricked, and I had such a situation develop - I couldn't use the receiver b/c SqeezeServer couldn't "see" it. If you ever have a problem with your receiver/and server and need to re-setup the receiver as if new-out of the box, you can't do it with the iPeng and the Touch.
The official line from Logitech support is that the controller is required to setup the receiver. However, there is a plug in for the Slim Server software that allows you to access your receiver using command line functions (a "hack") and set it up without a controller, but you need to be comfortable installing it and using it by writing command line functions from your PC/MAC.
Just thought you should consider this before you sell the controller. You might want to wait till the Logitech Touch comes out (May?) - then you won't need the controller and you can sell both.