RE: 1/2a Piper J3
Mike, I suspect the weight was your enemy. 24 oz is a bit heavy for that size of model. The Herr J3 is about the same size as the 1/2A Texaco models and while I wouldn't call them a hot aerobatic model such models will fly and climb at a fairly sporty pace provided the weight can be kept under 20 oz.
Encan, if you made it up and covered it with something light (perhaps some of the new light films) and only use clear or no dope, depending on the material, you will have a lovely and lightweight flier that will provide all the slow training that you could wish for. Perhaps go for the smaller tank initially and then switch to the Golden Bee for longer flights. Built to 18 oz there is no reason you can't even learn to do basic loops and a few other tricks. It'll need to dive a bit for speed but that's OK.
And when the engine quits if you have a bit of height you can even learn to thermal soar with it. Just don't let it get downwind at all in the glide. It doesn't take much breeze to totally halt any forward progress with these things. And they don't penetrate the wind well even with the nose down a bit either.
You managed to fly that other model in the videos just fine from what I saw so the Cub will be a piece of relaxing cake that will look fantastic in the air.