RE: What is the difference in these two aircraft
The RC versions of the J3 and Super Cub ( PA-18) should be the same.
As already mentioned the biggest differences are the cowl and flaps.
There are 3 two place tandem Cubs, J-3, PA-11 and PA-18.
The J-3 has wooden wing spars and the fuel tank in the nose, because of this it has to be soloed from the rear.
It had a 65 or 75 hp engine without an electrical system
The PA-11 has metal wing spars with either one or two wing tanks, it can be soloed from the front.
The PA-11 did not come with flaps, it has a 85 or 90 hp engine and usually had an electrical system.
It had a cowl very similar to the Super Cub and is easily mistaken for one.
The PA-18 Super Cub has a larger engine than the PA-11, started at 135hp and went to 150 hp.
It also has an additional air scoop under the cowl and electrical system was standard.
Also some more instruments started showing up on the instrument panel.
Some very early Super Cubs didn't have flaps, I don't know if these were modified PA-11's or actually labeled PA-18s.
Now having said all this, the J-3 and PA 11 has been around a long time. Many people recovering or repairing their airplanes have opted for updates. I flew a J-3 with metal spars.
I've seen some PA-18s with the engines exposed, usually towing banners.
Didn't come with them originally but someone changed them a long time ago.