If you want to build a kit for the experience, then I think the Amp kit is a great idea, but since it's essentially the same price as the RTR, it's not a cost effective option.
Now, between the Ruckus and the Amp, it depends on how you want to use it. I use both, and for most things I prefer the Ruckus. It's bigger, with a lot more ground clearance, and that makes it more versatile. It can go almost anywhere, and definitely does better on rough terrain and in grass. It doesn't handle as well as the stadium truck chassis, but it still handles pretty well for a big wheeled RC. If most of your driving will be on pavement or smooth dirt driveways, then the Amp will probably do better. Another consideration is that the Ruckus comes with the much better Spectrum controller, where the Amp comes with a goofy looking no name controller. That said, the Amp controller works just fine.