The next step is usually to get a good aileron trainer so that you can learn to fly the 4-channel planes. The best plane for this task used to be the Multiplex Minimagistar (Minimag), but a recent newcomer, the ParkZone T-28 Trojan, is even better. I've also seen some people go directly from the SuperCub to the ParkZone Typhoon2 or the ParkZone Stryker-C.

I've never owned a Stryker (mostly because I won't fly a plane that can't do ground take-offs) but I've seen others fly it and its a very nice plane, particularly in handling some wind. I've been flying the Typhoon for nearly two years and its a terrific plane. I recently bought a T-28 Trojan for a more relaxed flying experience compared to the Typhoon, and I'm amazed at how well it flies. For your situation, I would recommend the Trojan.