I'm not sure exactly what a Slow Stick will train you to fly for sure. If you're very confident in your abilities, you might try the new Parkzone T-28 Trojan. It's a 4-channel sport plane with a brushless power system. It should slow down well enough to land, but should be pretty fast with the throttle wide open.

If you're not wildly confident in your piloting abilities, I'd suggest you move up to a Hobbyzone Super Cub RTF. It's going to fly faster than your Slow Stick, but still be a good trainer aircraft.

If you'd rather build another GWS ARF instead of buying an RTF package, the new GWS AT-6 Texan would be a good choice. It would be a nice stable aircraft but faster than your Slow Stick and far more aerobatic.